Sound problems "may" be from a pinched wire in the hinge of the system. I've noticed a distorted DS chime when I initially clean and refurbish DS lites. Generally the chime goes back to normal after I've fixed the issue I had that caused me to open it up.
In regards to the freezing, Nintendo basically suggests that you try the game in another DS lite to troubleshoot if it's the cartridges or the unit. If the games freeze on other systems, then it's the game. If it's the unit, then you'll want to replace the logic board.
If it's the unit constantly freezing then most likely it's a bad IC and those are going to be a '''huge''' pain to replace, nearly impossible without the proper equipment. Luckily eBay has a bunch of "for parts" units and you could look into searching the descriptions of DS lite units that'll fit your needs to get a replacement board.