ello, Casey. My Catalán is a bit rusty, but as no one else has answered, I’ll share with you what I remember from “Teach Yourself Catalán” and also from conversing with Catalonians (many years ago). In any event, this answer will hopefully bump your question to the top of the page long enough for a native Catalonian speaker to notice.
A and E: When unstressed, both of these are considered to be a schwa. This even happens in diphthongs. In reality, this supposed schwa sounds more like a weak /æ/ in Barcelona (there, you may even hear /a/, like /ɛ/ in the West (Lleida, Andorra, etc), and like /e/ in Valencia. In the Balearic Islands, maybe a real schwa. In most places (except maybe the Balearic Islands), it would not be a good idea to use /o/. I won’t mention Perpinyà.
Exception: The E sounds as /e/ even when unstressed if followed immediately by A. Example:
O: When unstressed, sounds like /u/.
I and U: These two vowels do not lose their timbre, although they may be a bit shorter in unstressed position.
Anyhow, keep these ideas in mind as you do your listening practice, and you should quickly notice what’s going on for yourself.
I've just noticed the second part of your question. Yes, there are rules. Similar to Spanish, but combinations like ia count as separate syllables (so a word like "volia" doesn't take an accent). Catalán also makes use of the diaeresis a lot more than Castilian.