3DS XL speakers: better fidelity (improved, about 1.45x the ammount of khz the main 3DS speakers), through headphones, the same.
DS Lite speakers: about 0.75 out 1 (compared to 3DS XL) in terms of volume, which is definitely lower than normal 3DS (about 1/3 on the 3DS XL out 1, being normal 3DS).
fidelity, 0.4 out 1 (compared to 3DS XL, again)
Yes, you can change the surround settings to stereo to get more volume, but it's not that much. I don't remember if the sound on the XL is louder than in the DS Lite but I'd guess that the DS is louder.
People usually complains about the volume on the XL because they got used to the volume on the normal 3DS, at least, that happened to me.
I find my DSi being louder than my regular 3DS and a friend's DS Lite, but I never tested it versus a DSi XL or 3DS XL, but remember people posting about the DSi XL being louder than the regular DSi, aside from that the DSi has a better sound chip so the audio is better than previous DS models and I fell the 3DS speakers sounding as clear as the DSi ones but with less bass.