I tested many different DLNA apps on my Android smartphone. Most of them are completely unusable either because they don't detect either my NAS or the NP3900 or because they can't play my audio formats or because they run far too slowly (in some cases, because my library has hundreds of albums) or because they are generally too buggy.

The only app that seems (seemed) useful was The 2Player Network Player.

Alas, after using 2Player to start playing tracks from my Synology DS211 NAS the NP3900 sometimes freezes. See video: <http://youtu.be/zVU4BiAFlwk>. In this video the tone you hear is a tone from the song that the NP3900 was playing when it froze. Note the interesting pattern on the display.

Whether or not 2Player contains bugs, the NP3900 should never, ever behave this way.

2Player app is dated 15 August 2011.

Serial number printed on the bottom of the NP3900: PT1A1130002295
From the settings menu:
"NP3900/12"
"Software version: V G5.32S"
--
Thomas Hood