Regardless of what you use, no-one is going to sue you unless your end product is worth something (except perhaps in North America, but might be mitigated if you take it on TV). Most Game Studio projects don't need to worry. Even if your game is massively successful it will probably work out at less than $1 per SKU.

I would avoid wma because it is owned by Microsoft and they might find a way to make it so your sound clips won't play on Windows if Micosoft don't want them to.

You can probably just use mp3. If you are sure you are going to sell ten or hundreds of thousands or you are worried about it, use ogg. Just remember to make your installation program intall it. Don't forget, like me).