No sound coming from Left earphone - Any suggestions?

---

Sound only comes out of the right headphone ( I've seen in another post that this has happened to others, a suggested 2.5mm to 3.5mm adapter was the fix. Surley the headphone which comes with it should work properly!!) Anyway got the adapter and it did not make any difference.
I've tried buying new headphones but still no difference. One interesting thing though, I tried my friends nokia headphones and it worked ok but these headphones had no volume control.
The 2.5mm jack has three black plastic rings i've noticed some have two....
Can anyone say for sure what I should purchase to get over this problem??