As stated in my review, 688i's sound effects are compressed in a format that modern Windows OSes can no longer read natively. The AGG file must be decompressed in order for the sound effects to play normally. The SC Sound Utility was designed to extract the WAV files from the AGG compressed format in all of Sonalysts' sims (688i, Fleet Command, Sub Command and Dangerous Waters). Below is the link on the SUBSIM forums to download the utility. You must register to download the file, but there are no other obligations on the forum beyond signing up.

To decompress the sound files, select sfx.agg in the Audio directory from the utility's menu, select all of the files in the archive and select decompress all to do just that. The game's sound effects will play normally after this is completed.

SC Sound Utility hosted on Subsim.com Forums

Thanks, it works for me! :)