Download GraphEdit.
GraphEdit | Software - Digital Digest
Drag .mkv of the movie in to GraphEdit and see what it shows.
It should look similar to this.
Also go in to the ReClock Configuration. Advanced Settings. Near the bottom there is a drop down box, select your media player and select Load always. When you open a media player for the first time after ReClock is installed a dialogue box will pop up, you have to make sure you select 'Load always'.
Are you even using a 6.1/7.1 sound setup? If not there is a much easier fix than all this. If you are using Media Player Classic or MPC-HC. Go to options, externasl Filters. Add Filter... Then add madFlac Decoder. Now you should see madFlac Decoder in the list there. Double click it to get to the filter properties. Click the box that says 'limit output to max 5.1 channels'.
If you don't have MPC installed than you are going to need a different program to get to the filter properties.
Radlight Filter Manager
Open the program, click DirectShow Filters, scroll down to madFlac Decoder, click the property page. Click the box that says 'limit output to max 5.1 channels'.
Either of this should work.