The game sound engine is fully 3D positional. For this reason, you won't hear sounds that are played if you are more than 75km from the sound source. e.g a ship exploding.
Also, make sure that your speaker configuration is correct. e.g. if you have a 4-speaker setup, make sure that you have the settings for your sound card are configured properly in control panel. If you have powered Dolby 5.1 (or 6,1, 7.1) speaker setup, make sure you have them configured correctly as well.
If you are still having problems with hardware audio, you can force use of the software audio by add /x to the game's shortcut.
