So if the mitsubishi remote has Sony audio codes listed, then you only hope is to switch the sony to AV1 codeset, which the oem remote isn't needed when switching codesets. On the receiver itself:

On the Receiver:

1. Press I/O to turn off the power
2. Then, press I/O while pressing ENTER (On the STR-DA2100ES, press I/O while pressing INPUT MODE)
3. Each time you press I/O the Command Mode switches between AV1 and AV2 (initial factory setting is AV2)

Once it is in Av1 mode, reprogram the directv remote:

1. av1 or av2
2. press and hold MUTE & SELECT, wait for two blinks, release
3. enter 3 1 0 5 8

and try mitsubishi sony codes.