Convert MKV file's audio

[frosse]

With Coreplayer and CoreAVC now being available for OSX I can finally play my 1080p MKV-movies without lag on my macbook.

...though whats sad is that Coreplayer does NOT support AC3-audio. Most MKV's have AC3-audio.

Now what I need to do is to convert the AC3-audio to something else, like mp3 or AAC.

Coreplayer supports the following audiosets:

Audio: MP3, AAC, MKA, WAV, OGG, Speex, WAVPACK, FLAC, MPC, AMR, GSM, ADPCM, ALaw, MuLaw, MIDI, WMA

Is there a software out there that can convert the audio without loosing video quality or audio sync AND save the output file to mkv?

I've tried:
Handbrake - which doesnt import MKV.
VisualHub - which doesnt save to MKV
FFmpegX - which doesnt save to MKV either.

This sucks.

Please help me!

Regards
Vic