The page mckinley linked has a further link to the DB website
. Section 6 describes services for disabled people.
Section 6a mentions preconditions such as Schwerbehinderten-Ausweis and a Wertmarke that is available from the Versorgungsamt.
As I understand it people with some degrees of disability get the Wertmarke for free, others with different degrees of disability may have to pay a fee.
On the DB website the"Mobil mit Handicap"
page has a downloadable brochure
In the brochure, page 20 explains how disabled people can travel for free on bus and train services. It states...
The disabled person needs a Schwerbehinderten-Ausweis and a Wertmarke.
Disabled people with those do not have to pay in Busses, trams, S-bahn or on Nahverkehrszüge (RB, RE, IRE) trains.
Disabled people cannot travel for free on Fernverkehrszügen (D, IC, EC, ICE, TGV, RJ, CNL, EN)
So, under certain conditions a deaf person with a Schwerbehinderten-Ausweis might be able to travel on some trains for free. In other cases they might have to pay for the Wertmarke but from then on tickets would be free.
I'm sure someone will correct me if I'm wrong.