This is just a guess, but I would say that because you do not have true stereo, but dual-mono, the reciever is mixing those to the Center channel. What make/model is the surround receiver? Maybe someone knows it, and can comment on how it is treating the dual-mono.
The DVD player is a Sony BluRay and the receiver is an Onkyo AV receiver HT-R380.
My thinking is that it is something that I am doing wrong inside Premiere vs something that the DVD player or receiver is doing. My final product will need to play on a variety of DVD players so I need to get it right.
It just seems odd that my default output DVD provides sound to the left speaker but then removes it when I fill Left.
Am I setting up my audio tracks wrong? Or should I being doing something with a 5.1 track???
Let me ask one question to start.
Is it normal to just have audio on the left or right when it is imported and then have to use the "fill left" or "fill right" to get audio on both sides? Is that the way it is normally done?
Fill Left or Right is the way I normally operate and have had no issues with it. But I've also never found a need to bother with a 5.1 mix. Have you checked Premiere's Audio Mixer settings to be sure each channel is assigned to an appropriate submix and if so have you positioned the tracks' balance "puck" to L or R? If you haven't read it yet, check the help file for 5.1 Panning and Balancing.
I frequently use 2 two mikes, one on each channel. I map them to mono, then bring them into a sequence with two mono tracks. My master is stereo. I don't need to do anything except adjust the volume to balance the tracks.
My point: your audio mapped to mono, brought onto a mono track, master track set to stereo.
But I don't see where your workflow has gone wrong.
audio now only comes from the CENTER speaker. I dont get anything out of left OR the right speaker
That is normal and the way it should be.