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You want to prioritize VoIP traffic at congestion points, which for you, looks to be whenever your LAN bandwidth hits your 6 Mbps WAN.
If the carrier takes your 100 Mbps, and doesn't just generally police all traffic to 6 Mbps, but instead has different priority queues for the 6 Mbps, all you should need to do, is insure you traffic is correctly processed by your WAN vendor. This, though, might require marking your traffic for the WAN provider.
If you carrier first polices all your traffic at 6 Mbps (many do), then you need to shape the traffic (with you own prioritization) before the provider "sees" it. If you need to "shape" on the 3750, there's a command to limit a port's bandwidth utilization, as a percentage. However it's not exact, so you may need to "shape" slower than 6 Mbps to insure 6 Mbps won't be exceeded. (BTW, for 6 Mbps, if you need to do QoS, you would be better off with an ISR.)
You also mention 6 MLPPP, but it's unlclear what your device is for that.
How to configure 3750 QoS is involved. Basically when you enable QoS, each port has four dedicated egress queues. By default, different traffic markings go into one of the four queues, each queue has same share of the bandwidth, and almost same share of the buffers. The PQ command you noted, enables the first queue to always transmit its packets first. Normally, you'll want to do that for VoIP bearer packets, which you've (insured are) directed to that queue. You also want to insure that VoIP signally packets are not likley to be dropped and, more or less, are forwarded quickly.
Cisco has multiple papers on QoS configurations, including papers for 3750s, so instead of my trying to present that, the above is intended as an overview. Feel free to post additional questions, the more specific, the more likely you'll get an answer.
