Sorry for the late update Tascam DM3200 users who've followed my post (Problem when selecting mute on channel 1, 2, 3, 4 on all layers on DM-3200).
I recently solved the problem about a month or two ago but had other issues in studio, mainly #1 studio computer. (Had to replace pc motherboard, (board about 3 years old), had some kind of bios issue. Bios would not hold the month of July and September for 2015, most problematic).
Anyway, the problem with my DM3200 was due to a bad copper connection on circuit board (P/N E95188700A, (assign pcb assy for DM-3200)) around #1 mute button, (I think moisture corrosion).
While looking for burnt or discolored components on board using a pair of watch repair magnifier glasses I noticed the bubbling or flaking of the white protective coating protecting said circuit.
Repair Steps:
1-Removed flaking coating, found discoloration on copper circuit.
2-Proceeded to remove all flaking coating of said circuit beyond the circumference of the discolored area. A little pasted the point of the normal colored copper area.
3-Blew away excess, cleaned area with a tooth brush and 50% isopropyl alcohol, dried area and surrounding components well with hair dryer set to cool/warm for about an hour to be completely sure.
4-Applied rosin soldering flux to exposed circuit, then applied a bead of solder spreading over discolored and normal colored exposed copper until completely covered.
5-Problem solved, I have my board back, o' happy days!
I believe the problem was due to moisture in room during the early summer which was when the problem first started. So far haven't experienced any other issue in studio other than main computer motherboard bios, which also may been due to moisture. Now I keep the AC on until I get a dehumidifier.
Happy Mixing!