Download Audacity here. Also download the LAME MP3 library here
Install Audacity on your system. Install the LAME MP3 library.
Click on File > Open... and select the mp3 file in question
Click on Track > Stereo track to Mono
Click on File > Export. Choose "MP3 Files" as Format on the dropdown menu and click save.
Note 1: If you want to save ...
I did some research, and came to the conclusion that you can't assign the control knobs to any of the native parameters in GB(it doesn't allow MIDI Mapping). Your only option if your daughter really wants to control EQ and other things with the Oxygen's contol knobs, is to use third-party plugins with MIDI assignable parameters.
you could definitely use ffmpeg for this kind of job.
$ ffmpeg -i path/to/your_input.aiff path/to/the_converted_file.mp3
ffmpeg is a huge framework dedicated to multimedia encoding.
Once installed write down the above command line on your terminal,
this should convert your mp3 to the default parameters of ffmpeg for mp3.
For more infos about options you ...
On Linux systems like Ubuntu, you can use ffmpeg:
ffmpeg -i input.mp3 -c:a libmp3lame -q:a 2 -ac 1 output.mp3
-c:a libmp3lame: The audio codec to use
-q:a 2: The audio quality (bitrate), see LAME Bitrate Overview
-ac 1: One audio channel
Since most laptops have only one input (usually for microphones, not line-in) nowadays, it would be safer to have an 2 channel usb or firewire interface. It should have phantom powered inputs anyways so you don't have to bother with external ones.