well I'll try to summarize as best as I can...
1. RPGXP uses 4 types of "channels" for sounds, BGM, BGS, ME, SE (Background Music, Background Sounds, Menu Effects, Sound effects)
Basically it's possible to have BGM and BGS play at once, however, both the BGM and BGS are the only ones (I think) that loop,
should you call ME and SE, they only play once and stop.
now then, what I'd do in your case, is have the forest music (assuming it's music you want to keep throughout the entire map)
under BGM, then as you approach the beach, have the BGS handle the beach's sound effects (I'd draw a line of player touch triggered
events to start/stop the BGS depending how close you are to the beach)
2. Afaik, it's highly recommended you know to sprite if you want some quality sprites that actually fit in your game, however you can always
check for spriting sites (spriters resource comes to mind) to find sprites you might like and use them (give credit if they tell you to ;D)
Don't like to give credit? there's other sites that handle royalty-free sprites. but google is your friend for that one though.
P.S: We do have a sprite request section if you're interested (and patient enough)