any bit depths and frequency rates that might be available will be listed in the audio MIDI setup window, properties for: built in audio, source: optical input... the format and bit-depths should be available under source. Choose whatever suits you. I'm stuck on an ibook so i can't say what is available to you, but my built-in mic has settings for 48, 44.1, and 32, with 24 or 16 bits.