Same problem here. HR24 DVR. When researching online it seems this is a problem that Direct has been dealing with since June 2010. No fix yet from what I have been able to find out. A MAJOR inconvenience.
UPDATE:
Starting over the weekend, DirecTV started to raise the volume levels on the SD channels, in response to all the feedback they have been getting in repose to the volume equalization project.
Local SD levels are expected to be adjusted up approx 5dB, with a target completion date of Monday 23rd.
National SD levels are expected to be adjusted up approx 5dB, with a target completion date of Friday 20th.
These are target dates, so it's possible that the levels, particularly those for some peoples local SD channels, may take a little longer to be increased.
No, it was not a software download. The adjustment was made at the DirecTV location where the signal is processed before being sent to the satellite.
I hope that this information is accurate.