I have looked at what I think is the important part in the log; UMS analyses the file as mp3 and the characteristics within but does not seem to select the right engine to start playback.
So is it a UMS issue after all that has never been tested before with Xbox?
I have highlighted the part in the log (because the log file is huge):
013-08-26 17:54:44.927 [New I/O worker #2] Opened request handler on socket /172.10.1.40:54543
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Request: HTTP/1.1 : POST : upnp/control/content_directory
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Matched media renderer "Xbox 360" based on address /172.10.1.40
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: Connection: Keep-alive
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: Content-Length: 582
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: CONTENT-TYPE: text/xml; charset="utf-8"
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: Host: 172.10.1.10:1337
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: SOAPACTION: "urn:schemas-upnp-org:service:ContentDirectory:1#Browse"
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Received on socket: User-Agent: Xbox/2.0.16537.0 UPnP/1.0 Xbox/2.0.16537.0
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Recognized media renderer: Xbox 360
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] HTTP: upnp/control/content_directory / 0-0
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] discover 0 refresh forced: false
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] discover 0$1 refresh forced: false
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] discover 0$1$7 refresh forced: true
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2]
Matched format MP3 to "06-sub_focus-rock_it.mp3"
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2] Matched format JPG to "des and jimi wedding.jpg"
DEBUG 2013-08-26 17:54:44.927 [New I/O worker #2] File automatically removed: Inglourious_Bastards.mkv
DEBUG 2013-08-26 17:54:44.927 [New I/O worker #2] File automatically added: 06-sub_focus-rock_it.mp3
DEBUG 2013-08-26 17:54:44.927 [New I/O worker #2] File automatically added: des and jimi wedding.jpg
TRACE 2013-08-26 17:54:44.927 [New I/O worker #2]
Matched format MP3 to "E:\Media\Movies\TEST\06-sub_focus-rock_it.mp3"
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2]
Parsing results: 06-sub_focus-rock_it.mp3 / container: mp3 / bitrate: 200778 / size: 6908959 / codecV: und / duration: 00:04:35.00 / width: 0 / height: 0 / frameRate: null / thumb size : 0 / muxingMode: null
audio: id=0 / lang: en / flavor: null / codec: mp3 / sf:44100 / na: 2 / bs: 16 / Sub Focus|Sub Focus|Rock It|2009|6
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] Adding 06-sub_focus-rock_it.mp3 / class: net.pms.dlna.RealFile
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2]
Getting player for 06-sub_focus-rock_it.mp3
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: MEncoder, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] AviSynth/MEncoder cannot determine compatibility based on audio track for E:\Media\Movies\TEST\06-sub_focus-rock_it.mp3
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: AviSynth/MEncoder, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] tsMuxeR cannot determine compatibility based on audio track for E:\Media\Movies\TEST\06-sub_focus-rock_it.mp3
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: tsMuxeR, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: FFmpeg, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: FFmpeg Audio, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: Audio High Fidelity, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: FFmpeg Web Video, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: VLC Web Video, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: VLC Web Video (Legacy), enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: MEncoder Web, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: VLC Web Audio (Legacy), enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: FFmpeg DVR-MS Remux, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: dcraw Thumbnailer, enabled: true, compatible: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: AviSynth/FFmpeg, enabled: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2] player: VLC, enabled: false
TRACE 2013-08-26 17:54:44.989 [New I/O worker #2]
no player found for 06-sub_focus-rock_it.mp3