Hi Paul,
Welcome to YamahaSynth!

The .X6A File format is a proprietary file format for the MOXF, only another MOXF can read that file. It contains no audio in a form that can be made into something that you can play for your friends. In order to create an Audio File you need to create what is commonly called a .WAV File.

A .wav File is what is created to burn commercially available CDs. It uses about 10MB of memory per stereo minute. You create it by recording the audio Outputs of the MOXF either to your computer via USB connection, or to an external recorder via either USB or the analog audio L&R Outputs.
An mp3 is created by compressing the .wav audio data so that it takes up less memory... you do trade some sound quality for the smaller file format... but you windup with a much smaller file.

You can create .wav Files from your MOXF Sequence Data by using the free copy of Cubase you received when you purchased your MOXF (assuming you were the original purchaser). I’m sure it came with an offer to add mp3 creation as well (if you took advantage of the time sensitive offer). But surely you can create the higher quality .wav File by setting up to record audio to Cubase AI.

You’ll need the MOXF, a USB cable to connect the MOXF “TO HOST” port to your Windows or Macintosh computer.
The MOXF has a built-in Audio/MIDI Interface which allows you record data directly to your computer via USB.
You will need to download and install the “Yamaha Steinberg USB Driver” for your particular computer type and operating system.

Please see the following tutorial:
MOXF Basic Audio Recording