R:\bin>ffmpeg -i extra.wma -acodec aac -vbr on  extra.m4a
ffmpeg version 4.1.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 8.2.1 (GCC) 20190212
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Guessed Channel Layout for Input Stream #0.0 : stereo
Input #0, asf, from 'extra.wma':
  Metadata:

    WMFSDKNeeded    : 0.0.0.0000
    DeviceConformanceTemplate: MP@ML
    WM/WMADRCPeakReference: 32767
    WM/WMADRCAverageReference: 9169
    WMFSDKVersion   : 10.00.00.3646
    IsVBR           : 0
    Agility FPS     : 1106231951
    encoder         : Lavf58.20.100
  Duration: 01:00:20.97, start: 0.000000, bitrate: 68 kb/s
    Stream #0:0: Audio: wmav2 (a[1][0][0] / 0x0161), 44100 Hz, stereo, fltp, 48 kb/s
Codec AVOption vbr (Variable bit rate mode) specified for output file #0 (extra.m4a) has not been used for any stream. The most likely reason is either wrong type (e.g. a video option with no video streams) or that it is a private option of some encoder which was not actually used for any stream.
Stream mapping:
  Stream #0:0 -> #0:0 (wmav2 (native) -> aac (native))
Press [q] to stop, [?] for help
Output #0, ipod, to 'extra.m4a':
  Metadata:

    WMFSDKNeeded    : 0.0.0.0000
    DeviceConformanceTemplate: MP@ML
    WM/WMADRCPeakReference: 32767
    WM/WMADRCAverageReference: 9169
    WMFSDKVersion   : 10.00.00.3646
    IsVBR           : 0
    Agility FPS     : 1106231951
    encoder         : Lavf58.20.100
    Stream #0:0: Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s
    Metadata:
      encoder         : Lavc58.35.100 aac
[aac @ 0000016b8f33a980] Queue input is backward in time
[ipod @ 0000016b8f3aedc0] Non-monotonous DTS in output stream 0:0; previous: 15360, current: 14333; changing to 15361. This may result in incorrect timestamps in the output file.
[ipod @ 0000016b8f3aedc0] Non-monotonous DTS in output stream 0:0; previous: 15361, current: 15357; changing to 15362. This may result in incorrect timestamps in the output file.
size=   57361kB time=01:00:20.87 bitrate= 129.8kbits/s speed=52.3x
video:0kB audio:56723kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.125517%
[aac @ 0000016b8f33a980] Qavg: 255.732