Its a common fault in our car. The potentiometer inside the heater control panel that controls the speed wears out over the years. If you know how to solder then you can try to fix it yourself. See here:
https://www.volvoforums.org.uk/showt...l+panel+repair
If you can't solder, then you can either look for a refurnished heater control panel on ebay, or you can send it over and I can do the repair