I think that the level of Audio experience will depend on the design of the game it is built on.
Here's a few examples I drew up on...
Doom3 uses barely any music, and the music that it does have is not as recognizeable and/or signature-riffic in a way. Instead, there is a lot of emphasis on the sound and sound environment. I think this was done to add to the realism factor. That being said, there are a few sparse tracks that are played when one expects a "BIG" battle to happen.
Megaman2 had limited sound effects, related to triangle and square wasves and that "Clunk" when one hit and destroyed an enemy. That being said, Megaman 2 has titles that have probably been remixed more often than any other (aside from FF7) - especially the Flashman theme. Of course, Megaman 2 needed music, due to the limitations in the audio hardware. If you get the chance, download a bunch of Megaman 2 remixes from http://www.ocremix.org
and play on an emulator - it's a blast
So there you have 2 different opposites.
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I think something for the music of FreeOrion would fall in between. For example, the galaxy music would be there as a filler... You could have your staple tunes, fade in and out with galaxy / planet sound effects, etc. Within those tunes, players could add / remove their own .ogg files to add extra variety.
As for Ship Combat, the mood and status changes. You now have the opportunity to code music with dynamic events - such as in Freespace 2 or Lords of Magic, or command&Conquer Generals. It doesn't lend itself well to a 4 minute song by the Barenaked Ladies.
As for Ground Combat, we'll see when it becomes fleshed out.
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I think what's important is to look at different scripting events in the game, and think "Ok, that's where a track could go" and "Ok, this event could spawn track XXX or YYY". This way future modders or composers could write their own Race / Track music and insert it into the game.
A game that did not do this well is Morrowind. While the music is really well written, all they scripted for was "Is the player walking around" and "Is the player fighting something". Had I been a part of the code-process, I would have suggested "Player in inn, Player underground, Player Dungeon, Player Outside, Player City, etc.." and "Small fight, big fight, boss fight, scripted fight, etc.. etc..." The creators of Neverwiner Nights did this - and the musical experience is superior... Despite the fact that both titles have the same composer
So in the end, I do think that FO should have its own Audio tracks. However, the player can be given the opportunity to add / remove tracks (such as in Total Annihalation) for certain track events (such as Galaxy music.). Otherwise, they'll just open an MP3 player and have their own music playing in the background.
Anyway, that's my beef.