Problem with an external mic and T2i
Could you link the products you are talking about.
When you say it can only play back on one headphone do you mean while you are recording or do you mean one channel when you are playing back the recording? AKA not Stero.
Have you tested the mic and cable with another sound recording device to see if it working as expected?
Are you sure the cable is fully plagged in? I have seen with some sound equipment you only get mono if not all the way plugged in.
I took a course in art last winter. I learnt the difference between a fine oil painting, and a mechanical thing, like a photograph. The photograph shows only the reality. The painting shows not only the reality, but the dream behind it.