Ergos: Starting a new game is indeed required, but, even if you do, the changes introduced by the mod are quite subtle, so maybe it's working as intended. You can check the author reply to a similar comment here:
https://forums.beamdog.com/discussion/comment/766090/#Comment_766090 Thanks, I've used it in the EE without any problems so I know what to look for, she says different things when you click on her when it's working than when it's not, that's how I can tell.
EDIT: I still get the error message when I try to install to a fresh Tutu with no mods except the Degreenifer. This is what my debug file says:
Installing [Chatty Imoen: Expanded Character Sounds] [v1.3]
Appending to files ...
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\data\DATA\DEFAULT.BIF]
BIFF may be in hard-drive CD-path [E:\Games\Windows\Baldur's Gate - TOS\DATA\DEFAULT.BIF]
[E:\Games\Windows\Baldur's Gate - TOS\DATA\DEFAULT.BIF] 5847739 bytes, 444 files, 0 tilesets
Appending [0x80101FEF CD_STATE_NOTVALID...] to [STATE.IDS] because it does NOT contain [CD_STATE_NOTVALID]
Appended text to [STATE.IDS]
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
Copying 1 file ...
[chattyimoen/sounds/english/ay#imna1.wav] loaded, 13612 bytes
Copied [chattyimoen/sounds/english/ay#imna1.wav] to [override/ay#imna1.wav]
[chattyimoen/sounds/english/ay#imna2.wav] loaded, 31944 bytes
Copied [chattyimoen/sounds/english/ay#imna2.wav] to [override/ay#imna2.wav]
[chattyimoen/sounds/english/ay#imna3.wav] loaded, 39708 bytes
Copied [chattyimoen/sounds/english/ay#imna3.wav] to [override/ay#imna3.wav]
[chattyimoen/sounds/english/ay#imna6.wav] loaded, 51812 bytes
Copied [chattyimoen/sounds/english/ay#imna6.wav] to [override/ay#imna6.wav]
[chattyimoen/sounds/english/ay#imnb2.wav] loaded, 99066 bytes
Copied [chattyimoen/sounds/english/ay#imnb2.wav] to [override/ay#imnb2.wav]
[chattyimoen/sounds/english/ay#imnbr.wav] loaded, 77376 bytes
Copied [chattyimoen/sounds/english/ay#imnbr.wav] to [override/ay#imnbr.wav]
[chattyimoen/sounds/english/ay#imnc1.wav] loaded, 75056 bytes
Copied [chattyimoen/sounds/english/ay#imnc1.wav] to [override/ay#imnc1.wav]
[chattyimoen/sounds/english/ay#imndn.wav] loaded, 70566 bytes
Copied [chattyimoen/sounds/english/ay#imndn.wav] to [override/ay#imndn.wav]
[chattyimoen/sounds/english/ay#imnhr.wav] loaded, 65838 bytes
Copied [chattyimoen/sounds/english/ay#imnhr.wav] to [override/ay#imnhr.wav]
[chattyimoen/sounds/english/ay#imnld.wav] loaded, 179648 bytes
Copied [chattyimoen/sounds/english/ay#imnld.wav] to [override/ay#imnld.wav]
[chattyimoen/sounds/english/ay#imnpp.wav] loaded, 58656 bytes
Copied [chattyimoen/sounds/english/ay#imnpp.wav] to [override/ay#imnpp.wav]
[chattyimoen/sounds/english/ay#imnr1.wav] loaded, 48420 bytes
Copied [chattyimoen/sounds/english/ay#imnr1.wav] to [override/ay#imnr1.wav]
[chattyimoen/sounds/english/ay#imnr2.wav] loaded, 33896 bytes
Copied [chattyimoen/sounds/english/ay#imnr2.wav] to [override/ay#imnr2.wav]
[chattyimoen/sounds/english/ay#imns3.wav] loaded, 28650 bytes
Copied [chattyimoen/sounds/english/ay#imns3.wav] to [override/ay#imns3.wav]
[chattyimoen/sounds/english/ay#imns4.wav] loaded, 24216 bytes
Copied [chattyimoen/sounds/english/ay#imns4.wav] to [override/ay#imns4.wav]
[chattyimoen/sounds/english/ay#imns5.wav] loaded, 19762 bytes
Copied [chattyimoen/sounds/english/ay#imns5.wav] to [override/ay#imns5.wav]
[chattyimoen/sounds/english/ay#imns6.wav] loaded, 50188 bytes
Copied [chattyimoen/sounds/english/ay#imns6.wav] to [override/ay#imns6.wav]
[chattyimoen/sounds/english/ay#imnsp.wav] loaded, 91872 bytes
Copied [chattyimoen/sounds/english/ay#imnsp.wav] to [override/ay#imnsp.wav]
Compiling 1 dialogue file ...
[chattyimoen/tra/english/ay#imoenp.TRA] has 1 translation strings
[chattyimoen/dlg/ay#imoenp.d] loaded, 138 bytes
Processing 1 dialogues/scripts ...
ERROR locating resource for 'REPLACE_SAY'
Resource [IMOEN_POST.DLG] not found in KEY file:
[./chitin.key]
ERROR: preprocessing APPEND_EARLY [tb#_compile_eval_buffer/chattyimoen/dlg/ay#imoenp.d]: Failure("resource [IMOEN_POST.DLG] not found for 'REPLACE_SAY'")
Stopping installation because of error.
ERROR Installing [Chatty Imoen: Expanded Character Sounds], rolling back to previous state
Unable to Unlink [chattyimoen/backup/0/OTHER.0]: Unix.Unix_error(1, "unlink", "chattyimoen/backup/0/OTHER.0")
[chattyimoen/backup/0/UNSETSTR.0] SET_STRING uninstall info not found
Will uninstall 19 files for [CHATTYIMOEN/SETUP-CHATTYIMOEN.TP2] component 0.
Uninstalled 19 files for [CHATTYIMOEN/SETUP-CHATTYIMOEN.TP2] component 0.
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
[./override/fw0125.are] loaded, 9760 bytes
[./override/ar6111.are] loaded, 2776 bytes
[./override/fw2003.are] loaded, 1684 bytes
Unable to Unlink [chattyimoen/backup/0/READLN.0]: Unix.Unix_error(20, "unlink", "chattyimoen/backup/0/READLN.0")
Unable to Unlink [chattyimoen/backup/0/READLN.0.TEXT]: Unix.Unix_error(20, "unlink", "chattyimoen/backup/0/READLN.0.TEXT")
ERROR: Failure("resource [IMOEN_POST.DLG] not found for 'REPLACE_SAY'")