As long as you trust the team and have contracts (make sure you have contracts!!!) you should be fine. I've seen and heard of musicians usually getting between 8 and 15%, depending on if it includes sound effects or not, as well as the number of tracks/themes that will be in the game. If it's a very music centric game then this number should of course be more. Talk to your team though and find an equitable solution, but just use contracts to protect yourself!
>>On the contract part<< Do take a look in the laws you have in your country about intellectual property.
I know in some country's the can still come back on contracts if it's not considered a 'fair deal'.