$ ffmpeg -y -i combined_audio.mp3 -c:a aac -b:a 384k -profile:a aac_low -strict -2 combined_audio_ytfriendly.mp3                                                ffmpeg version N-75976-gd59bfcd Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 5.2.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-lzma --enable-decklink --enable-zlib
  libavutil      55.  3.100 / 55.  3.100
  libavcodec     57.  5.100 / 57.  5.100
  libavformat    57.  3.100 / 57.  3.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 11.100 /  6. 11.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
[mp3 @ 0000000000593e60] Skipping 0 bytes of junk at 409.
Input #0, mp3, from 'combined_audio.mp3':
  Metadata:
    encoder         : Lavf57.3.100
  Duration: 06:39:57.94, start: 0.011995, bitrate: 111 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, mono, s16p, 112 kb/s
[mp3 @ 000000000059bbe0] Invalid audio stream. Exactly one MP3 audio stream is required.
Output #0, mp3, to 'combined_audio_ytfriendly.mp3':
  Metadata:
    encoder         : Lavf57.3.100
    Stream #0:0: Audio: aac, 44100 Hz, mono, fltp, 264 kb/s
    Metadata:
      encoder         : Lavc57.5.100 aac
Stream mapping:
  Stream #0:0 -> #0:0 (mp3 (native) -> aac (native))
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
[aac @ 00000000005b2f20] Qavg: nan