user6187: 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.

user6187: 
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.

6187: <h2>Synopsis</h2>

<p>I use Compressor to export my FCPX project to h264 .mp4 files. </p>

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

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

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

<p>Any ideas or solutions?</p>

<h2>Environment</h2>

<ul>
<li>Mac OS X 10.9.5 (Marvericks)</li>
<li>Final Cut Pro X 10.1.4</li>
<li>Compressor 4.1.3</li>
</ul>

<h2>Setting of Compressor</h2>

<p><img src="https://i.stack.imgur.com/eXTvZ.jpg" alt="setting"></p>

<h2>Output of ffprobe</h2>

<h3>audio of previous output (good quality)</h3>

<p><code>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
</code></p>

<h3>audio track of this time output (bad quality)</h3>

<p><code>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
</code></p>

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

<p><code>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
</code></p>

<p>However the problem was not solved.</p>
 6187: <ol>
<li>FCPX project -> Play through timeline = <strong>OK</strong></li>
<li>FCPX project -> Compressor -> ProRes422 = NG (muffled sound)</li>
<li>FCPX project -> Compressor -> AAC Mono = NG (muffled sound)</li>
<li>FCPX project -> Compressor -> AAC Stereo = NG (muffled sound)</li>
<li>FCPX project -> "Share" -> Master (h264) = <strong>OK</strong> (but the size is too large)</li>
<li>FCPX project -> "Share" -> Master (h264) -> Compressor -> AAC Mono = <strong>OK</strong></li>
</ol>

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