You need to use the pan command for mplayer to merge audio tracks. To use this in Smplayer, open your "preferences" window and select "advanced". In the "Run" dialog box add the following option:
-af pan=2:1:0:0:1:1:1:1:1:1:0:0:1
af ("audio filter") pan (the command that mixes audio channels)=2(#of channels you want it mixed to)
The next bit is more difficult to explain. Each audio channel is seperated into as many values as channels. in this case there is 2 so each channel is seperated into (volume of this channel in left speaker):(volume of this channel in right speaker). If your describing the left speaker's audio channel it would then be 1:0 which means (full volume to the left):(no sound to the right).
Each audio channel from the source is represented by it's volume level in each output channel.
For flac,ogg and multi-channel WAV it's; FL:FR:FC:LFE:SL:SR however for other codecs (DTS,ac3,aac,etc.) it can be different. The Mplayer manual states ac-3 as FL:FR:SL:SR:FC:LFE for example.
The value I gave above seems right to me but I havent used them all in isolation to be certian, feel free to mess around.
Some helpful resources I found were:
http://www.mplayerhq.hu/DOCS/HTML/en/ad ... nnels.htmlhttp://forum.doom9.org/archive/index.php/t-137259.htmlI hope this helps bud.