A few days ago the water in the mastered room bathroom went from freely flow fast to a slow dribble, after it had previously been running fine. The sink faucet made a noise like slurp,then no longer ran fully. Both hot and cold, turn fully on flow low to dribble. Also, not sure if it matters, but less than a month ago we had to replace our gas water heater. The new one had to have a thermal expansion tank, there has never been an issue with flow prior to this time. What reasons would cause this to happen?

a month ago we had to replace our gas water heater
Sounds like you might have disturbed the pipes and had trash. With the pipe walls.
Try this. Shut the water off to a affected valve and open it up. Remove the cartridge and check and clean the inlet ports. Now place a pan over the valve and turn the water back on for a moment to flush out the supply and the valve body. You get a full flow when you open the valves? Let me know. Back to you, Tom