Output via Compressor comes with bad quality audio (no problem via Share of FCPX) Synopsis

I use Compressor to export my FCPX project to h264 .mp4 files. 

Playing the file I exported today, the audio quality was bad (sounds muffled). This problem appears only in the output via Compressor.


Playing on the FCPX timeline -> OK
Playing the output exported via "Share" feature and "Apple Devices 720p" setting -> OK
Playing the output exported via Compressor -> NG


There were no problem two months ago. I don't make any changes in the export settings.

Any ideas or solutions?

Environment


Mac OS X 10.9.5 (Marvericks)
Final Cut Pro X 10.1.4
Compressor 4.1.3


Setting of Compressor



Output of ffprobe

audio of previous output (good quality)

Stream #0:0(jpn): Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 63 kb/s (default)
    Metadata:
      creation_time   : 2014-11-29 10:42:07
      handler_name    : Core Media Audio


audio track of this time output (bad quality)

Stream #0:0(jpn): Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 44 kb/s (default)
    Metadata:
      creation_time   : 2015-02-07 14:28:54
      handler_name    : Core Media Audio


Looking at bitrate above, it's less than one of the previous output. 
I retried the encoding with setting the audio bitrate to 128kbps.

Stream #0:0(jpn): Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 86 kb/s (default)
    Metadata:
      creation_time   : 2015-02-07 13:15:43
      handler_name    : Core Media Audio


However the problem was not solved.
FCPX project -> Play through timeline = OK
FCPX project -> Compressor -> ProRes422 = NG (muffled sound)
FCPX project -> Compressor -> AAC Mono = NG (muffled sound)
FCPX project -> Compressor -> AAC Stereo = NG (muffled sound)
FCPX project -> "Share" -> Master (h264) = OK (but the size is too large)
FCPX project -> "Share" -> Master (h264) -> Compressor -> AAC Mono = OK


These are sum-up of results in my trial and error.
The 6th trial resolved my problem, although it is not a perfect solution.