Very good.
I use my phone to watch Netflix all the time and the volume gets more than loud enough, I'd swear you can even detect a little stereo separation which is surprising. It's pretty clear too, the classical music ringtones Nokia included sound wonderful, I use them as an alarm clock in the morning. Nice to wake up to.
Could the speaker be better? Sure I guess, but it's the best speaker I've had on any phone so far. I think it would be cool to have stereo speakers, one at either end of the phone, but that's probably too much to ask and would make the phone even bigger.