Compressed formats are not really relevant to considerations about the bit depth itself. Besides, mp3 [audio] compressions have difficulty in handling specific

Compressed formats are not really relevant to considerations about the bit depth itself. Besides, mp3 [audio] compressions have difficulty in handling specific samples, or type of samples (eg. sharp attacks), and they may manifest artifacts independently of the bitrate; MP3, AFAIK, also has a ceiling of 320 kbps within the standard specification, which certainly doesn't help. Secondly, I'm not sure if you process further the MP3s (when you refer to mixing), but if you do, you're definitely going to make noticeable, artifacts which weren't so in the unprocessed MP3 form.