can anyone here tell me how to convert a "BIG file" to an AAC, AIFF, WAV, or an MP3?

I am not familiar with a "BIG" file.

Wotsit, which is fairly comprehensive, doesn't list a BIG format. What is this file? Where did you get it?

then google it.

it is a music file that my friend gave me. he can't open it and wants me to "crack the code" for him.

It's probably a proprietary format which is specific to whatever game it came from. Some audio editors will let you mess around with importing raw audio files; you might play around with that, on the chance that the format doesn't use compression. Other than that, you're unlikely to find useful information on this format.

Audio editors?

Like what?

then google it.

google what?

Use Google to find an audio editor.