We have a communal alarm system in the block of flats with sounders in the flat.  How often should the company who maintain the fire alarm quarterly check the sounders within the flats?

Can the sounders not be heard from outside the front door? thereby negating any reason to come in

No, a common areas only fire alarm system is useless as you will loose too much sound pressure (30db through the front fire door and wall, plus loss over distance)  from the common areas so sleeping residents will not be roused (nor someone in the living room with the telly turned up).

The flat hall sounders are there to give 85dB outside the bedroom doorway, the absolute minimum likely to rouse.

As far as maintenance goes, in respect of individual devices they only need to have been checked once in a 12 month period over a minimum of 2 service visits not exceeding 6 months