I was decently close with my biochem professor but wasn't close at all with my biology professor. A LoR is great when you can get someone to speak about not only your academic ability but your character, but it is hard to find that especially if you attend a big state school like I did. If you know them at all, they'll probably just look to see you did well on tests, attended classes, and turned work in on time and be able to craft a decently good letter. You can always ask and if they feel uncomfortable writing it, they will tell you. They won't write a negative one.

Don't get too caught up in volunteering. Again, if you could find time to get a hospital job, even PRN, it would show your dedicated to getting as much experience as possible but it is totally not a requirement. I wouldn't ask your volunteer coordinator for a letter because they probably don't know you well enough. But it is very unfortunate CASAA wipes all of that info, I remember having to get 3 new letters as well. They'll remember you applied last year and jumped through all the hoops to reapply, which is surprisingly important. Even as anesthetists, we are constantly jumping through hoops with credentialing, certifications, etc. Keep truckin, you'll be fine!