I've managed to solve all of my issues on my own. For anyone who finds this post with the same issues:
I'm using NAudio Version 220.127.116.11
I was receiving the “Buffers already queued on play" error because I had an event handler hooked up to the "waveOut_PlaybackStoped" event. The handler pointed to a method that was meant to handle looping, when the file ended, I'd seek to the beginning
of the file and restart the playback. The problem with this was that the event only fires when you've called the waveOut's Stop() method. Before I realized this was happening I used pause() instead of stop(), which bypassed the problem but created others with
data being left in buffers when I un-paused as well as the “Buffers already queued on play" error. Removing the event handler and using the stop method fixed all these issues.
When playing multiple streams at the same time, you'll need to convert them all to the same format. I used the "WaveFormatConversionStream" which cannot be disposed until you’re done with all the other streams involved with playing, seeking, stopping,
I also had issues with the seek method, I couldn’t get accurate positioning. The stream's "CurrentTime" property works better. I also use the streams volume property to change the volume for each track. Both of these gave me the flexibility to change
the position and volume of each stream instead of the waveOut object.
Hope this helps someone else...