You can always try taking a mono sum or the left channel output from your PC and putting into the other input on the Tascam and try recording it live that way as you play along, having both of the interface's inputs recording at the same time.
However, depending on what DAW you are using, it will probably want to use the Tascam as your ASIO driver to get the mic's audio into the DAW, which will disable the output from your PC's sound card.