Quality of sound is a VERY subjective evaluation. I was concerned about sound quality too and I went into a TMo store today and asked if I could checkout a Z1s even though I couldn't buy it until Wednesday. The rep was helpful and brought out what I believe was there training unit. Plugged in my Klipsch S4's and listened. I thought the sound with the stock player was fine, good enough for me to ask the rep if he could hold a unit for me. Since it was music I wasn't familiar with, time will tell. I really need to get Poweramp on the phone and test with music I am familiar with.
I do know that I haven't found a phone yet that sounds better than my Cowon dedicated media player. My 2 cents!