Are you trying to play these files with an extension .ac3 which contains 5.1 audio?
If yes, then your player does not natively support playability of these files. Instead you may need to convert these ac3 5.1 files to wav 5.1 and try. Even after converting them there is no loss in quality as wav's are uncompressed.
You can use foobar2000 software (free) for the same.
AC3 is supported as an audio stream in a container and not directly. The video part (whatever format) is interleaved with an audio format (ex. ac3 5.1, dts) in a container (ex. mkv, mp4, etc) and is decoded by the player.
Your player supports these audio files extension...
.aac, .mka, .mp3, .wma, .wav, .mp4, .m4a, .flac, .ogg
On a side note....
There is a firmware update available for your player, so I suggest you should upgrade it for better functionality.