The DGX-660 will not play .M4A audio files. It requires audio files in the uncompressed Microsoft standard 44.1 KHz 16 bit .WAV format. See "Audio files" on Page 38 of the DGX-660's Owners Manual. M4A uses a compression codec that the DGX-660's operating system can not decode. This is pretty much typical of most keyboards that play audio files, although there have been some models in the past few years that will play files in the MP3 format, and typical of most keyboards that play audio files, the DGX-660's display will not show/list files that it can not play.
The DGX-660 is a new model. It will most likely be a while before videos and tutorials begin being released for it, and these are almost always by third party vendors, often at less than professional quality. It takes time to produce a professional quality video tutorial. There are already quite a few entries for it in YouTube, but so far, most of these are demos and overviews, not tutorials. While demos and overviews may not show you "HOW" to do something, they can be useful in showing you some capabilities of your keyboard, that you did not realize it had. I did see one DGX-660 tutorial on YouTube that covered how to record with it. Just go to YouTube and enter "DGX-660 tutorial" in the search field. In the meantime, your best source of information for getting acquainted with your new keyboard is the Owners Manual that came with it. I recommend downloading a PDF copy of it, so that you can do electronic searches ("Edit/Find") for items of interest. It took me less than 3 minutes that way (including download time), to find the above answer for you. You can download a copy of the DGX-660 Owners Manual from:
https://usa.yamaha.com/products/mus...ble_grand/dgx-660/downloads.html#product-tabs