Lost Left Midi Signal on Audio Interface

it would be difficult to lose just left MIDI since the MIDI cable doesn't have R/L references (only pan data) and because MIDI is not sound, it's just data.  so, the problem is going to lie in what you're using for your synth output from the MIDI tracks, not the MIDI itself.

what are you using for your MIDI OUTPUT?  hardware synth?  softsynth?  if softsynth, which one?  If hardware synth, how do you have the audio cables connected and to what?

ok - sounds like everything is connected correctly (as it should have been if this worked before, I understand).

troubleshooting this can be tedious, but just go at it from a process of elimination.

1. check the tascam software mixer and see if the left input is muted and make sure the meter moves when you press a key on the keyboard.
2. verify the same thing in sonar.  make sure the left input from the tascam will respond with meter moving when the track is armed.  if not, verify the input is set correct
3. check your cables - not just make sure they're plugged in, but replace the cable path one at a time for the left channel in case one of the cables has gone bad.  replace the one from the keyboard to the input of the mixer, then replace the one from the mixer output to the tascam.  if you don't have enough cables to replace, just swap the ones from the right channel as a troubleshooting function and you can always put it back if it proves that the cables are good.
4. check your keyboard left output by connecting a speaker directly up to the left output and playing something.

that should get you started.

If the setup could allow a selection of fields to restrict the search, it would be a more useful function.

-----------------------------------------
Fruit Sclerometer

However, by going into "Audio" (I believe under the options tab) and de-selecting "Enable read caching" and de-selecting "Enable write caching" so far, seems to have solved all my problems.

Seems doubtful to me that this solved the problem.