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