@IrvSp Thanks for your response, but my Roku Soundbridge has been playing my entire iTunes library - both mp3 and m4a - for 16 years, and so did my previous Cisco/Linksys EA4500 router's media server, for the past 8 years. It is only the AX3000 media server that isn't playing the m4a files. In fact, I just unplugged the USB drive from the EA4500 router and plugged in into the AX3000, and now the Soundbridge plays only the mp3 files, not the m4a files. The only thing that's changed in this equation is the media server itself, i.e. the AX3000.
Also, i've never purchased music from iTunes. The only reason i have .m4a files, is from ripping certain CD's in iTunes.
thanks,
js.