A stronger argument. While it is not jarring to have multiple ship designs on the screen, you can only play one music track at once. By what criterion do you decide which species music will play? Is it going to feel any different to the player than randomly shuffling music tracks?
Well, there could be some way to determine the dominant species
in a fleet, either by the number of ships crewed or by some weighted sums (ship size or fire power could be a factor). And by 'dominant' I don't mean 'in charge', just 'most representative'.
That was my intention, yes. The client would pick music for either a representative species in his own fleet, or one in one of his opponent's fleets.
I don't see how the average player is form an association between music and the species.
Because the (my?) plan is for each species' main theme to play when you're in diplomacy with them (and yes, that idea suffers from the conflation of species and empire as well; everything is fine if a species can have diplomacy as one of its "roles", otherwise things need a little more re-thinking). It should be obvious to the player the species to which a particular battle theme belongs if he's heard that species' main theme before.
It might require a bit more work/thought to keep the selection of music from seeming arbitrary in some cases, for example when your opponent has different species on diplomacy from the one that pilots most of his ships, but I don't feel like that's a major problem.