Oh ok, I see now, Thought you were just asking for the program. Personally I would never bother examining the code of such product because for me and I assume for others too perhaps, its a waste of time. Not because the program is bad but because it does something that's already been done with other programs/ways and offers nothing extra.
See most of those "converters" they use standard free libraries/code to read/write file formats and they use windows codecs for conversion and just add their own user interface. If you've seen (or heard) one, you have seen them all, assuming you use same quality parameters. Some rare times they finetune audio compression via presets or let the user finetune the compression parameters (bitrate, sampling rate, Engine quality/speed ratio etc) and maybe those can make some difference. There is enough source code out thee to even make your own version if you know how to code.
Dont take my word tho, try to convert with VLC, and compare the outcome with what you use now, You might be surprised. Yes VLC supports any to any conversion (use google). Its not the easiest to use but it will do the job and for free (remember to set same quality standards for both). Then again with today's audio players, you don't even need conversion. Most of them, if not all support either mp3 or m4a among others, so whats the point of converting.
We also need to remember that you really want to avoid converting one compressed format to another when possible because there is always loss of quality. With today's modern music it might not be so easy to detect due to the limited audio range of the songs, but you can really feel it with older songs that cover broader audio spectrum.
Apologies for drifting a bit out of subject but to conclude, my advice is, if you want to Medicines a program for cracking sake, by all means go for it. Its fun and time will pass fast. If you do it out of need for a quality conversion... you will find that audio editors like Steinberg Wavelab or Adobe Audition (both can be found here) might serve you better as I think both have batch audio convert support too and you can also edit/clean/enchance your songs if you ever need. If you want fast and easy conversions then any of the prgrams above will probably do the trick for you.