The best audio convertor out there is dbpoweramp. It's free, and there's an OGG plugin(and many others) available in their Codec Central. It has a swell little UI and is easy to work with. There's also a PowerPak that costs $14. There's a 30-day demo for it and you can only use the LAME MP3 feature during that time or after registering.
If you're going to be needing something to rip temp/royalty free audio tracks for your game then the only thing you should be using is Exact Audio Copy.
