The most likely explanation for the left channel not working on the phones jack is that the tip contact on the internal socket has got bent. Have you got any way of testing the L and R outputs on the rear of the US-122L? Would you feel confident about taking the cover off the interface with it disconnected from everything and having a good look at the jack connector? You could see visually what happens when you then insert the phone plug.

If your soundmapper will let you take audio input from one device and send output to another, you could use the computer's soundcard as the audio output, with appropriate loss of quality on replay. Because you would still be taking the input from the US-122L, there would be no change in the quality of what was recorded on disk. The soundcard is only the last stage in the output, so any FX applied in the DAW software would not be affected by swapping to a different output system. It may sound different, of course, but that would be as a result of the quality of the output device and not the signals out of the DAW.

I have experience with Tascam gear... and while I used their pro series tape machines without trouble, most of their digital gear has given me many hair-pulling moments.

To add to what Bos suggested...

Have you done software (driver) or firmware upgrades to make sure you have the latest versions?

Have you tried using a third part driver, like Asio4all? Tascam's drivers can be inherently flakey... Not the most solid drivers ever developed.

Have you checked to see if you have solid stereo output from a built-in sound card?

Your Tascam drivers may be conflicting with any other audio drivers you have installed on your system, like Realtek, Soundblaster or any of the other audio devices that are fairly well suited for gaming but not so well-suited for actual quality recording. You may want to uninstall any of these factory installed drivers, or at least disable them in Control Panel.

Are you having any issues with "stuttering" or freezups while recording using the Tascam? This could point to buffer settings but could also point to a driver conflict.

I'm not saying that these are the root of your issues, Boswell's advice is certainly solid because if you do in fact have a short on the left channel output then you would need to address that regardless of other issues you may have in terms of driver conflicts.

Thanks for info. I tried using the RCA outs on the back which worked for a day or two but the Left started going out again. I would save taking it apart for a last resort because putting electronics back in their cases can be a pain sometimes. Also I'm not sure how to fix it, bend contact back into place maybe? But since the back LR output also does it I'm not so sure the contact is the cause. "The soundcard is only the last stage in the output, so any FX applied in the DAW software would not be affected by swapping to a different output system" I figured that this was how it worked but wanted to make sure before spending hours mixing. It won't allow me to record with US122 and output through soundmapper at the same time but I can just switch back and forth from recording to mixing. I tried what Donny said about disabling all other sound drivers but that had no effect. I am getting stereo output from my internal soundcard. I'll try Asio4all, I wasn't aware of third party drivers for audio interfaces. I'm hoping that's the problem because its drivers have caused me headaches in the past, that's the main reason I didn't uninstall and try reinstalling them before posting.

So you have a left channel cut out on all outputs? Headphone and line/monitor out?

I dunno Andrew, this is starting to sound less like a driver issue and more electronic/internal connections, as Boswell suggested.

I'm assuming you have already tried playing an instrument or singing through a mic and monitoring that signal in real time with phones or monitors as opposed to listening to recorded playback?

Well, that's in line with what I expected. Plugging headphones into the front panel TRS jack on those units is designed to cut out the signals to the individual rear-panel RCA line outs (don't ask me why), and it must do this by NC contacts on the front panel jack. If the tip contact strip on the jack socket is bent sufficiently that it does not connect with an inserted plug, then it's more than likely that the corresponding sprung NC contact is also not connecting, and hence no L signal to the rear panel.

Plugging headphones into the front panel TRS jack on those units is designed to cut out the signals to the individual rear-panel RCA line outs (don't ask me why),

which is very odd, Bos... because I have a Tascam 1641 and it doesn't do that. With cans plugged in the front jack, all outputs on the rear - CR monitor, line outs, etc., will all continue to pass signal. Go figure.

I agree - it's inconsistent. From the US-122L handbook:

PHONES jack
(stereo 1/4" phone) This is a headphone output jack.
NOTE
If you connect headphones to this jack,
no signal will be output from the LINE
OUT jacks.