In case this is useful info:
I have a rather sizeable drive plugged into my Mondeo: my music sources are invariably from YouTube, converted from .mpeg4 (which is video-plus-sound) to .mp3 (audio only) using Real Player's own converter, divided by me into files, and...thats it. Occasionally I may copy music off my Android phone, no probs as it's already in .mp3. Create a file on the PC desktop called "converted files" and stick it all in there.
Then, you might find if you are lucky that your car will bluetooth your PC if parked very near to the house, otherwise you have to bring the hard drive indoors and load it up when plugged into the PC.
My car's reproducer very occasionally hiccups, perhaps because of the size of the hard drive, and needs switching off and being allowed to calm down, but the system on the Ford couldn't be simpler.
There are a large number of software programs out there which will convert pretty well anything to anything else: just find the one that's easiest to use.
Good luck !