[What can be done to reduce the volume when playing at home?]

I said key, but pitch is probably the correct idea. I play along with YouTube music that in in tune with my Morse Anglo. 

[Which is suitable for me?]

What kind of music do you play? The Anglo is generally considered to be the choice for Irish  music. 

[Morse Concertinas]

I have a Morse and love playing it. 

[What can be done to reduce the volume when playing at home?]

When I started playing, I over pushed the bellows and everything was loud, in particular the  chords! I have found that playing along with a YouTube video set to a low volume helps me control my volume. It makes me play soft enough to hear the speakers. Other times, I'll up the speaker volume and increase my level. Going back and forth from time to time has helped me play with clarity at a lower volume. 

I suggest that you look for a YouTube version of a tune in the correct key. 

Crag Road, McHughs and The Mist Covered Mountain is one example.