what is in the iso file? is it just an image of an audio CD? if so, just mount the iso with daemon tools iso mounter, so the image shows up as a virtual drive, then rip it with whatever you use to rip CDs to MP3s, preferable EAC (exact audio copy)
I shrunk the scenes I wanted with dvd shrink, and it made an iso file, so I guess it was both audio and video, but I just want the audio.