My m4a are acquired exclusively from iTunes store during the last 12 months, so I must presume they are under DRM...
And I did not have the minimum of a problem, playing them on my PC and MC/DC-Bass (1.1.1) until I have installed build 35. - Build 32 was still perfect and build 35 no longer. All there, the same settings, the same system.
The MC version is 15.0.32 below, because I've allready tried to step back, as mentioned earlier.
My system:
Media Center 15.0.32 Registered -- C:\Program Files (x86)\J River\Media Center 15\
Microsoft Windows Vista 6.0 Service Pack 2 (Build 6002)
Intel Pentium III 2127 MHz MMX / Memory: Total - 4192 MB, Free - 1844 MB
Internet Explorer: 8.0.6001.18904 / ComCtl32.dll: 5.82.6001 / Shlwapi.dll: 6.0.6002 / Shell32.dll: 6.0.6002 / wnaspi32.dll: N/A
Ripping / Drive E: Optiarc BD DRV BC-5600S Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 100
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive E: Optiarc BD DRV BC-5600S Addr: 0:1:0 Speed:16 MaxSpeed:16 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: Yes
Use playback settings: No /
Portable Device Info
Removed devices:
Interface Plugins:
last.fm (Active)
TiVo Server (Active)