I don't know about the zero bytes problem, but I do know something about where the chipmunks might be coming from. In some players that play inside browsers (particularly flash-based), if the sample rate you used when you created the MP3 file is not divisible into 44.1 kHz, you'll get chipmunks. I use 22.05 kHz - it sounds pretty good for messages. I think 11.025 is also theoretically possible, but would almost certainly sound like the pastor is talking on an old-fashioned telephone because all the high frequency sounds would be gone. I hope this helps. You can Google something like "chipmunk mp3 flash" to find tons of discussions on the topic.
Another tip for saving bandwidth but still getting decent sound for spoken messages is to save them in mono instead of stereo. 22.05 kHz, mono, and about 32kb/s bitrate sounds quite good, and if you don't mind a bit of compression noise you could get away with 24kb/s bitrate. How to force mono varies by software, but it's something to consider.