I have a 1056B01, and it sends the Vol signal to Audio and then sends a signal using the Cable device, whether using your RMIR file or after a 981 reset. However, as it was delivered to me, it simply sends the Vol signal on the Audio device, which I assume is what you want. The difference is in the 1877 setup code. Apparently the built-in CBL 1877 setup has Device 28 OBC 60 assigned to "button" Phantom 10, and that's what the remote sends, although I don't know why.
The remote I have was delivered with an upgrade for setup code CBL 1877, and this upgrade doesn't have anything assigned to Phantom10. So try
adding this upgrade to your remote. Your remote will use the upgrade in preference to the built-in 1877.
I imagine there is some way to stop this behavior (short of burning some memory with an upgrade) via some 9XX command, but I don't know.
I also see that neither DVD or VCR actually perform a volume punchthrough. You can however add keymoves to these devices so that they send the vol signals to Audio.