1. If I use -ac2 the only difference I get is that it now reports "auto_resampler", not the explicitly called "Parsed_aresample":
Code:
[auto_resampler_0 @ 0000000000169a00] [SWR @ 0000000000169c40] Using fltp internally between filters
[auto_resampler_0 @ 0000000000169a00] [SWR @ 0000000000169c40] Matrix coefficients:
[auto_resampler_0 @ 0000000000169a00] [SWR @ 0000000000169c40] FL: FL:0.414214 FR:0.000000 FC:0.292893 LFE:0.000000 SL:0.292893 SR:0.000000
[auto_resampler_0 @ 0000000000169a00] [SWR @ 0000000000169c40] FR: FL:0.000000 FR:0.414214 FC:0.292893 LFE:0.000000 SL:0.000000 SR:0.292893
[auto_resampler_0 @ 0000000000169a00] ch:6 chl:5.1(side) fmt:fltp r:48000Hz -> ch:2 chl:stereo fmt:s16 r:48000Hz
2. I use an input file that ffmpeg detects as 5.1(side), whereas your file has back channels according to ffmpeg. I don't think that this is the reason behind the difference, but it should be noted. My ffmpeg has been built less than two weeks ago, when has yours?
3. My first test was with using aresample like this: -filter:a aresample=ocl=stereo