Thanks for your reply and advice.
We've already try 9.4 MR3 and 9.5 MR2 versions but we still have the same issues. So we've also tried 9.6 MR1.
We applied the recommended settings (no tunnel WLAN, OFDM-only disabled, bss-minrate = 5.5 Mbps, dedicaded voice vlan, directed multicast disabled) and we noticed an improvement but still have some interruption during soft phone calls.
We also noticed that interruptions could be due to backgrounds scannings, so we disabled it for the voice WLAN.
So to resume, we have indubitably better communication than before but still have some interruption during softphone calls.
If somebody has any other ideas...
Thanks in advance !