Hi Francois-Xavier, I believe Matrox Technical Support has contacted you on July 4th with some recommendations. If however, you are still having issues, please do not hesitate to contact us:
http://www.matrox.com/video/en/support/mxo/contact/
Re: MXO 1 and ATI 4870 HD by Andy Mees on Jul 16, 2009 at 6:08:02 pm
For those interested, a patch was recently released that resolves the audio distortion issue with the MXO and ATI Radeon 4800 series video cards ... see here: