DS speakers - not sure exactly, but they aren't great that's for sure. I use it for music, but i'm not an audiophile and i'm told that the sound output isn't particularly good either.
Slot 1 vs Slot 2 (generalisations)
Slot 2: earlier cart trend, requiring a passme device (in slot one). most can play gba roms and homebrew too (check as there are a couple of exceptions). some very old homebrew (not much worthwhile) only works properly on these carts. NDS rom playback is usually sufficient, but not as perfect as slot one carts (alot of newer games have problems). Some have ram which can be utilised in a few homebrews (usually optional and not super useful).
Slot 1: the newer trend. usually 100% or close compatible with nds roms. Fat read and write tends to be faster. keeps your slot two free. cheaper. newer technology.
As for going all out barring any price restrictions. I'd probably go for a slot one and a slot two cart.
Slot one has some tight competition; For ease of use, speed (And dual memory) i'd go acekard RPG (from my limited experience, although this cart is new to the market i'd wait it out a bit longer). For a larger and more advanced list of features (cheats, soft reset etc) and still with excellent speed, i'd go CycloDS. M3Real (also new) and Supercard one (menu speed a bit slow) are also solid competitors.
Slot two; the stand outs to me are M3 lite perfect (nice menu), Ezflash (great hardware, less perfect software, great gba compatibility).
(although considering you can emulate gba on psp and the ram isn't particularly useful at this stage in time, in your specific case i would ignore it)
I'm sure i'm missing some important points. i'm just a little tired and i'll try and help some more tommorow, if indeed no-one else can be of help (i'm sure they will)