Hi,
Thanks for the cool product.
Just for your information, I've tested your flowgraph on a GNU Radio Windows version.
It can generate the WAV-file, but I cannot play it via Dream, getting the error.

Environment: GNU Radio Companion 3.7.11.1 (installed from Pothos SDR), Dream 2.1.1.
PS: On Ubuntu all works ok, problem is in Windows version.