I too am now having a problem with audio on my MED500X with 4.0.4 firmware as described: can hear background, music, etc... but no voices. As if mixdown didn't occur and only one of 6 channels is audible.
I reverted to 3.0.5 firmware .... problem solved.
Is this a bug or something up with the audio in that mp4 file? What if I run it through a video transcoder (ie. handbrake) with different audio settings? If so, what audio is preferred?
The application 'MediaInfo' gives me the following information about the MP4 file displaying the problem...
General
Complete name : /Movies/How_The_Grinch_Stole_Christmas.mp4
Format : MPEG-4
Format profile : Base Media
Codec ID : isom
File size : 2.40 GiB
Duration : 2h 10mn
Overall bit rate : 2 634 Kbps
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L5.1
Format settings, CABAC : Yes
Format settings, ReFrames : 5 frames
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 2h 10mn
Bit rate : 2 185 Kbps
Maximum bit rate : 18.1 Mbps
Width : 1 280 pixels
Height : 544 pixels
Display aspect ratio : 2.35:1
Frame rate mode : Constant
Frame rate : 23.976 fps
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.131
Stream size : 1.99 GiB (83%)
Writing library : x264 core 80 r1376M 3feaec2
Encoding settings : cabac=1 / ref=5 / deblock=1:0:0 / analyse=0x3:0x113 / me=umh / subme=8 / psy=1 / psy_rd=1.0:0.0 / mixed_ref=1 / me_range=16 / chroma_me=1 / trellis=1 / 8x8dct=1 / cqm=0 / deadzone=21,11 / fast_pskip=1 / chroma_qp_offset=0 / threads=12 / sliced_threads=0 / nr=0 / decimate=1 / mbaff=0 / constrained_intra=0 / bframes=3 / b_pyramid=0 / b_adapt=2 / b_bias=0 / direct=3 / wpredb=1 / wpredp=2 / keyint=250 / keyint_min=25 / scenecut=40 / rc_lookahead=50 / rc=2pass / mbtree=1 / bitrate=2185 / ratetol=1.0 / qcomp=0.60 / qpmin=10 / qpmax=51 / qpstep=4 / cplxblur=20.0 / qblur=0.5 / ip_ratio=1.40 / aq=1:1.00
Material_Duration : 90324
Material_StreamSize : 2136466160
Material_FrameCount : 187523
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 2h 10mn
Bit rate mode : Variable
Bit rate : 445 Kbps
Maximum bit rate : 644 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Side: L R, LFE
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 415 MiB (17%)
Title : Imported with GPAC 0.4.6-DEV (build 1)
Encoded date : UTC 2011-08-18 09:38:55
Tagged date : UTC 2011-08-18 09:39:52