Thanks to Akuma and his thread on Vogons, I managed to enable adlib music in Fiendish Freddy's Big Top o' Fun.
www vogons org/viewtopic.php?t=76607

First you need a "Sound.com" file, if you have one laying around in another game folder that might work. Otherwise just download Akuma's SoundDriverPack from his thread and place one of the sound.com files in the pack in the same folder as the game. Any of the ones in the pack seems to work so I use the one in the 151 folder.

Next you need to edit the dosbox_fiendishFreddys.conf in the same folder.
Just add this below [autoexec] without the quotation marks.
"
cls
mount c "c:\path to game"
c:
cls
sound.com
freddy.exe -adlib
"
I'm not very proficient with dosbox so there might be a better way to do it, but this worked for me.

Lastly you need to edit the PlayFiendishFreddys.bat in the folder, just right click the file and choose edit from the menu.
Change
".\dosbox_windows\dosbox "FREDDY.EXE" -conf "dosbox_fiendishFreddys.conf" -noconsole -exit"
so it reads
".\dosbox_windows\dosbox -conf "dosbox_fiendishFreddys.conf" -noconsole -exit"

And there you go, Adlib music.
It's my first "guide" so hopefully I haven't missed anything and it's understandable.

Removed, my mistake.

Great! Glad to know that it worket.