For audio files we prefer to duplicate from a known good master disc and whatever format you have on your disc will normally be the format in which it is duplicated.
Although mp3 is a very popular audio format we still recommend you deliver finished product to your customers in .wav or .cda format so that it will play on any CD player or computer. There are enough people who still aren’t mp3 savvy who will cause you customer services problems if you deliver your audio content to them in mp3 format.
Now, the advantage of mp3 is the files sizes are considerably smaller, so you can get a lot more audio content on fewer discs, which will save on the overall cost of your product. However, just be sure you clearly identify the discs as mp3 discs for your customers to lower customer service issues.