Hi guys,
As some of you know from the other thread, I'm the developer of FlipBook. I'm really excited to see all the great things people are saying here! Just to let you know, feel free to contact support at flipbook.tv or use our contact form on the site if you have any feedback, bug requests, or feature requests. It's hard to keep track of all the places people talk about FlipBook (and it's especially hard when people leave feedback in iTunes reviews where I can't contact the person and address any problems).
Anyway, we are working on a mix of improvements right now, including trying some different performance optimizations. While I personally doubt I'd every have enough time to draw a movie that is supposed to play back at faster than 12fps on a phone (a tiny, 5 second movie at 24fps means 120 frames to draw), we'd certainly like to offer it. I'm also trying to find an OpenGL ES/CG/Shark expert to help do some performance tuning to see what we can do (if you know someone, please have the person email me). It's sometimes hard for me to find enough time in the day to handle everything myself

Keep in mind I'm all for pushing the boundaries of what the iPhone and iPod Touch can do while keeping the app's interface as easy to use as possible!
We've certainly thought about sound, and honestly the hardest part is figuring out the right way to do it. Your best bet right now might actually be bringing the video from FlipBook into iMovie or Windows Movie Maker and playing there.
Just to let you know, we are releasing a 1.0.1 upgrade to fix an autosave bug (sometimes the autosave doesn't fire properly, and I believe we've identified and fixed the root cause) later this week.
Also, I'd really appreciate it if you like FlipBook, if you could spread the word, post a review on iTunes, etc.. As of right now, every dollar we've made has gone back into the app, and the more sales we have, the more we'll be able to provide for free, such as more hosted movies on flipbook.tv.
As a last request, let's try to keep things civil. Somehow, the Flickbook user base has suddenly developed a huge following of folks with similar usernames who claim FlipBook is bloated and impossible to use yet are never able to mention specifics (and every review that says it has too many features also makes sure to mention Flickbook). There have been some nasty reviews posted to both apps talking about that. While I can't speak for the Flickbook group and their approach, I can say that I have never supported someone posting a bad review for an app that they didn't purchase and actually give a fair chance to. Now if you do purchase FlipBook, dislike it/find it hard to use, get in touch with me, and I'll see if there's some way I can improve the app.
Have fun drawing!