Please help me out, I use blender 2.76b, I tried to combine three actions in NLA editor but only the selected track is functioned while the others are muted. Please help me.
The nonselected actions are muted when editing the selected action. Try switching out of edit mode.
Similarly, if the NLA stack is muted when editing the action, the NLA Track below it will be edited with solo enabled.
If switching between NLA Tracks, the solo status for the previous track will be transferred to the new track.
If you've already considered that, then the speaker icons may be disabled: