For quite a while, I had this problem where out-of-game sound from other applications (Firefox, Teamspeak, etc.) would be reduced whenever I join an empires game. The sound would be ~ -3db (about half) what it was before I joined the game. If I was to restart the application (e.g. a media player) while I'm in-game, the sound would play fine UNTIL there is a map change, then the sound is reduced again. None of the in-game volume adjustments affect it (as expected) and I see no changes to the volume for any of those applications. I don't know if this is a feature of the mod/engine, similar to snd_ducktovolume when you use your mic which can be adjusted, or a bug. By the way, this problem is exclusively in Empires mod as I have tested it on other Half-Life 2 mods.
I have heard similar complaints about other games. Im sure you have windows 7 then. Windows 7 does it automatically because it detects that you are no longer paying attention to it so it lowers the sound. There was a way to disable that but I don't remember atm.
Right click the sound icon in the bottom right. Click Playback devices. Click the Communications tab. Click in the "Do nothing" circle. Click Ok ?????????????????????????????? Profit.
I have done that before, yet nothing has changed. Keep in mind that this is an unique issue with Empires mod. I have NEVER had this issue with TF2, Portal, Borderlands, etc. I noticed when you are in Empires and enter Options or Controls, the background music for empires is temporarily reduced by a similar amount. I'm not sure if both situations are related.
I was able to fix the problem a few days ago, though I'm not 100% sure what was the fix. I believe it was when I changed my Speaker Config (Options->Audio) from "2-Speaker" to "Headphone" and back to "2-Speaker" (I usually use a bluetooth headset). Now Empires does not knock down the volume of other applications. The in-game sounds were initially loud after these changes so my game/music volume are now around 10-15% from full volume. Thanks for all of the suggestions.