https
/esupport.sony.com/CA/p/model-home.pl?mdl=DAVTZ130&template_id=1®ion_id=3&tab=manuals#
That's the manual. There is no way to input 5.1 sound in it. It only has a stereo L/R input, and thus would only ever input.
"Press SURROUND repeatedly during
playback to select a surround audio output
mode. The selected mode appears in the
front panel display.
“MULTI” (A.F.D. MULTI): When the
source is 2 channel, sound is output from
all speakers.
“STANDARD” (A.F.D. STANDARD): The
system discriminates the sound format of
the source and presents sound as it was
recorded/encoded.
“DOLBY PL”(PRO LOGIC): Performs
Dolby Pro Logic decoding.
“2CH ST” (2CH STEREO): All formats are
downmixed to 2 channels."
You were probably in MULTI mode before and now you're in 2Ch. Switch to MULTI mode for fake surround. It's all the device will ever do with an input.