Provided that they're not packaged and that they're not encoded in a proprietary format, you would need to decode them, resample them with a lower bitrate and frequency, hoping they're not low enough already (or you would get a terrible quality...), if the codec allows it compress them, and finally re-encode them for the game.
You could do a similar process with textures, slightly compressing them. Loading will be slower if the game manages to read the compressed files, but you'll have a much smaller installation.
Be my guest if you want to get your hands dirty, however, as I literally spoke by theory only. I don't have the slightest idea on how to do that