A UPnP Media Server (such as Whitebear) publishes tracks to the player's Content Directory. The information for each track, contains a list of one or more urls by which the player can download and play the track. In the case of Whitebear, it offers in this list, a url for the track in native format, a url for the track transcoded into pcm format, a url for the track transcoded into MP3 format (insofar the native format is not MP3), and a url for the track transcoded into flac format (insofar as the native format is not flac). And the player may choose which of these it shall display and/or play.I'm curious to know how your client managed to get the HDR T2 to even list the non-MP3 audio files.
As we know, the Humax can only play MP3. So the trick is to ensure that there is always at least one offer for MP3 format for each track. Some players (and I don't know if the Humax is one such) only look at the first url in the list, and if this is not MP3 then the track does not show up at all. Against this, Whitebear has compatibility settings for each player type, whereby you can force MP3 to be the first shown format in the list.