I know that this is an old thread. But I had the same problem. This was on the Olevia FAQ page for this model
Direct TV Remote Codes 506 or 127
Direct TV (Remote Model RC-64RB) Remote Codes 11144, 11240, 11331, or 11610
I just got the HR21-700 with a RC-64 model remote and code 11610 worked for me. I can now change the volume and everything. No more using multiple remotes.