How can I use .m4a files in a MonoGame Windows Phone project?

I created .m4a files with Audacity and I want to use these audio files as sound effects in a MonoGame Windows Phone project. I used the MonoGame Pipeline tool to create the .xnb files, but I always get this error message:
Couldn’t find a default importer for “C:/Users/…/Game6/Game6/Content/soundeffect.m4a”
The settings of the file are:
Build Action Build
Importer [missing] [null]
Processor [missing]

What can I do so that the .m4a file works in a MonoGame Windows Phone project?

Hmm looks like m4a is not supported in the content pipeline (can someone confirm?)

So I guess there is no way to make them work in the default Monogame, you would need an importer for that. Ogg and Mp3 work fine.

I need an audio format that runs on Windows Phone an iOS. In addition, I don’t want to pay license fees. mp3 is not free.

ogg is not supported on WinPhone?

Plain old .wav always works, these files are just a tad bigger

ogg is working. I tried it with ogg sound effects and an ogg song. Will ogg sound effects and songs work on iOS as well?