mp3 player in C

http://stackoverflow.com/questions/428884/how-to-play-mp3-files-in-c

Easy way: Use a library (fmod is used in link above)
Hard way: Write your own decoder (here be dragons etc)

Most modern devices like smartphones etc have special hardware onboard to perform the decoding, so there would be a system call to make use of this. Doing it via CPU would use a lot more power on a phone. PC is so powerful that decoding is no big deal tho.

Oh, you can write static libraries in C++ btw (and mix it with C). So that isn't a valid reason for choosing only C. If you are limited to C because it's an embedded platform with limited toolchain then that would be a more valid reason.

With some kind of compiler option.... It depends what compiler you use. Here is how to do it for gcc: http://www.adp-gmbh.ch/cpp/gcc/create_lib.html