Audio CD images are not supported by WinCDEmu. My personal opinion is that storing audio CDs in ISO format is inefficient, as you cannot index separate songs, waste twice more space (compared to lossless APE format, if you are picky about quality) or 10x more if you accept MP3s with a reasonable bitrate, and have to mess with drive letters every time. Moreover, technically, emulating audio CDs is a non-trivial task (would take even more time than adding support for data NRG/MDS/CCD). So, if anyone submits a patch supporting audio CDs, I won’t mind including it, but I won’t develop such feature myself, as there is an easier and more smooth solution.
I recommend checking out various Audio CD ripping tools instead.