If it were only the internal speaker, there might be a fix. (If you're in areas where there's iron filings or powder flying around [like a construction site where they're drilling I-beams], the speaker gets "loaded". You can use a magnetized screwdriver to "sweep" the filings off the speaker cone.) If an external speaker is also lower in volume than it was, I suspect that some component is going bad, so the fix is having it repaired.
As far as an amplified speaker goes, the best one is the Motorola (any vintage is pretty good). You can usually find some on flea bay. Radio Shack also used to have a pretty decent one that was reasonably priced. They still have one RadioShack® Amplified Communication Speaker but it's not what I'd call reasonably priced.
I am sure you checked but could you scanner be set for wide-FM and you are listening to a NFM signal. Still find it difficult to believe modern solid state electronics would loose volume. Most times it's either all or nothing.