YANBU, I think you should be able to use it anywhere you can buy a ticket.
I've just looked at the terms and conditions.
www.disabledpersons-railcard.co.uk/help/railcard-terms-conditions/
The clause that matters is this one.
Tickets for your journey should be purchased before boarding the train. When buying tickets in person from a member of staff, you must show your valid Railcard. If you are buying tickets online or from a ticket machine you must declare that you have a valid Railcard by selecting the Disabled Persons Railcard option. If you fail to do so, you and, where appropriate, the adult travelling with you will be required to pay the full price Standard Single fare for your journey as if no tickets where purchased before starting the journey and in some cases a Penalty Fare. This does not apply if:
there was no ticket office at the station at which you began your journey or if the ticket office was closed, and
there was no working ticket machines from which you could buy discounted tickets, or
you have a disability which prevented you accessing ticket retailing facilities
In these cases you will be able to use your Railcard on train or at your destination.
If you can't hear sufficiently to use the ticket counter, the only way they could get out of this would be by arguing you could use a machine. So my previous question still holds.