Coming from Nexus 6P I can tell you the speaker on the XL is not as good.
I've never had a bottom speaker phone for a long time, had the 6P, had a Droid Turbo before that, then the Droid before that ... So I've always had front facing speakers.
I've already noticed that if you cover the speaker with your pinky, as I hold one handed and it's a natural spot for a pinky to sit, you really distort sound from playback ... It's not just muffle it's an odd distortion.
If you don't cover the speaker accidentally, it's not that bad, but not nearly as loud as 6P but it is more crisp and clean. Putting Google Play equalizer on doesn't lower the volume either, like it did on 6P, it actually enhanced the sound but you can't do bass boost or surround since it's not a stereo speaker.
Keeping the volume at max also doesn't distort sound like it does on other phones.
Bottom line ... It's not the best, prefer front facing, prefer dual stereo speakers for surround sound, but it's quality is better than most. The speaker is my biggest complaint right now.