$ ffmpeg -i test.m4a -i test.m4a -map 0:a -map 1:v -vf "setpts=N/(25*TB),loop=-1:32767" -shortest -c:a copy test.mp4
[...]
frame=  304 fps=0.0 q=28.0 Lsize=     106kB time=00:00:10.00 bitrate=  87.0kbits/s speed=59.9x
video:12kB audio:85kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 9.961186%
[libx264 @ 00000000005d7ee0] frame I:2     Avg QP:14.16  size:  3514
[libx264 @ 00000000005d7ee0] frame P:77    Avg QP: 9.34  size:    24
[libx264 @ 00000000005d7ee0] frame B:225   Avg QP:12.67  size:    16
[libx264 @ 00000000005d7ee0] consecutive B-frames:  1.3%  0.0%  0.0% 98.7%
[libx264 @ 00000000005d7ee0] mb I  I16..4: 67.0%  0.0% 33.0%
[libx264 @ 00000000005d7ee0] mb P  I16..4:  0.0%  0.0%  0.0%  P16..4:  0.1%  0.0%  0.0%  0.0%  0.0%    skip:99.9%
[libx264 @ 00000000005d7ee0] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8:  0.2%  0.0%  0.0%  direct: 0.0%  skip:99.8%  L0: 0.0% L1:100.0% BI: 0.0%
[libx264 @ 00000000005d7ee0] coded y,u,v intra: 18.6% 17.0% 15.1% inter: 0.0% 0.0% 0.0%
[libx264 @ 00000000005d7ee0] i16 v,h,dc,p: 82%  5%  7%  5%
[libx264 @ 00000000005d7ee0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 50% 23% 17%  2%  2%  2%  1%  2%  1%
[libx264 @ 00000000005d7ee0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 00000000005d7ee0] ref P L0: 84.8% 15.2%
[libx264 @ 00000000005d7ee0] kb/s:8.14

$ ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_frames test.mp4
[STREAM]
nb_frames=253
[/STREAM]