I have a Lumia 620 myself, and it's not too shabby at all. Quite a few review sites have maintained it as the champion of budget phones, alongside the 520.
The 620's speakers are nothing special, gets a tad bit shrill at higher volumes. Haven't tested out the earphones, but I'll assume that it's as average as it can get, give or take a few points. It does the job of a music player well enough, and as a bonus has a pretty decent back camera as well as a front camera, which is rare for budget phones. Honestly the biggest draw of the phone is the OS. Absolutely love it. Fast, sleek, has all the apps and games I'll ever need.
There are a couple sites you could use as reference for comparing the 620 and 520:
http://www.youtube.com/watch?v=53_rKwtFIkA
http://www.phonearena.com/phones/compare/Nokia-Lumia-520,Nokia-Lumia-620/phones/7745,7593
If you're willing to spend a couple more bucks there are better alternatives as well(though not necessarily better value for money), such as the Lumia 720. It's a gorgeous phone, really. Reminds me of the 800.