The PC version of San Andreas has a complicated history. The Steam and Rockstar Games Launcher versions received updates over the years that removed certain songs due to expired licensing agreements (such as songs from Tom Petty, Rage Against the Machine, and James Brown). Many players download the original audio files to restore the classic radio experience to the "definitive" versions of the game.
Launch the game and check if the "Options > Audio Setup" menu allows you to adjust the Radio and SFX volumes. Common Fixes for Audio Bugs
If you lost your original audio due to corruption, check if your game files can be verified via Steam/Rockstar Launcher before hunting for downloads.
If you are missing cutscene voices or radio music, the standard solution is to replace your existing audio files with those from the original v1.0 version of the game. Backup Your Current Files