It looks like you've been screwed, wrongly, I might add.
Sorry for your loss, first and foremost.
If the issue was a radio, that is a piece of equipment and thus a maintenance issue which would fall under the control of the airline. In that case, here is what their contract of carriage says in your situation. I'll provide a link afterwards so you can verify.
C. Schedule Changes, Delays, & Flight Cancellations within Delta’s Control
When, as a result of factors within Delta’s control, you miss a connection due to flight delays,
your flight is cancelled, or a substitution of equipment results in a change in the class of service
that you purchased or prevents us from transporting you, Delta will provide you with the
following:
1. Transportation to Your Destination
Delta will transport you to your destination on our next flight on which seats are available in
the class of service you originally purchased. At our sole discretion, we may arrange for your travel on another carrier or via ground transportation. If acceptable to you, we will
transport you in a lower class of service, in which case you may be entitled to a partial
refund as set forth below. If space on the next available flight is available only in a higher
class of service than you purchased, we will transport you on the flight, although we reserve
the right to upgrade other passengers on the flight according to our upgrade priority policy
to make space for you in the class of service you originally purchased.
2. Full or Partial Refund
If some or all of your ticket is unused, you may be entitled to a refund. Any refunds will be
made as provided in Rule 260.
3. Amenities for Delayed Passengers
If your travel is interrupted for greater than 4 hours after your scheduled departure time as a
result of flight cancellation or delay on the date of your travel we will provide you with the
following additional amenities to accommodate you during the delay:
(a) Meals
We will offer a voucher for meal service to each ticketed passenger.
(b) Hotels
If overnight accommodations are available at Delta contracted facilities, we will provide you
with a voucher for one night‘s lodging when the delay is during the period of 10:00 pm to
6:00 am. Delta will provide free public ground transportation to the hotel if the hotel does
not offer such service.
DL's Contract of Carriage Domestic
If they are unwilling to reimburse you, please contact the DOT at
this link to make a complaint and hopefully they'll kick DL in the wallet so your money comes back to you.