I work with electronics- and 37 years on the road with rock bands and my motorcycle riding has affected my hearing.( just a bit!).
My solution would be to tap into the existing alarm and use a relay to trigger another device- whether a "shaker' or a strobe light...which would be more "universal", changing frequency of the siren/buzzer/beeper wouldn't do much for those who are deaf.
This would not be difficult or expensive.
smoke and c/o alarms are readily available at retail, so propane seems to be the one which needs to be addressed
Mike