A/C and Heater has low volume of air being pushed through the vents?

Do you notice the blower fan changing speeds with each increase or decrease in setting? If not, you most likely have a failed blower motor resistor. If the fan speed changes, then there may be a restriction or blockage in the housing or duct work.

We once had a similar issue and the technician finally found plastic grocery bags stuffed in behind the vents.

Thanks for the update. It sounds like you may have a restriction somewhere. Sometimes the fins in the heater core and evaporator coil can get plugged up, especially if the vehicle was run a lot in dusty conditions. You can check airflow with a smoke pencil or incense stick. Run the blower on high, recirculation set to fresh air and hold the smoke near the recirc inlet on the passenger side under the hood. Look to see how fast the smoke is pulled in. Or you could remove the blower motor and inspect for dirt, debris, etc. If you have a borescope, you can snake it through the vents and look for a problem. Let me know what you come up with.