@joes44 ,
I had been an avid Foobar2000 (F2k) user and like that software.
F2k can play mp4. You might need a codec for that or maybe F2k can do it natively - I have never done that. But do you know that you can directly play Youtube playlists using the foo_youtube component.
Since you are new to F2k, here is a link to an excellent F2k setup page. BTW, ASIO and playing Youtube files are completely different topics. Not sure why you have ASIO in the title. I don't think you have to convert your mp4s to WAV to listen to them using F2k.