Hi!
I just connected my Cosmo (running the v25 Android build) to a Grundig 46 VLE 7239 WR television set (a rather basic model from 2013) using the Planet USB-C to HMDI adapter cable and a regular HDMI cable. Both devices were on when I connected the cable, but no audio was playing. Without doing anything in particular, except turning the volume all the way up on the Cosmo, I could then play sound via the TV. I tried both the pre-installed YouTube app, as well as a mobile sort-of-DAW (Caustic) and had no issues with either.
That, with a sample size of one, just says that it can work, but doesn't say much about why it isn't currently working for you. The following is, thus, just me thinking in text. If you have been able to get HDMI sound output on the TV from other devices, using the same HDMI cable (not the Planet adapter), we know that cable is good. If you have been able to get HDMI sound via the same HDMI connector on the TV, we know that connector is good.
As the behaviour changed when you removed and reconnected the cable, presumably while playing audio, that might suggest there is some protocol handshake involved that, under certain circumstances, can give different results. Perhaps the Cosmo can't reroute an audio stream to HDMI while it's playing? If so, that would likely apply, even if some (potentially misbehaving) app is effectively outputting silence. Just to make sure there isn't something in the background trying to play sound prior to connecting the cable, I'd try rebooting the Cosmo.
If possible, try another TV or a computer monitor with speakers and/or a headphone output. Just to be sure, check that the connectors are in good condition. Especially the USB-C connectors has a tendency to gather pocket lint. If you have a suitable magnifier, it might be worth checking the male USB C connector (in the Planet adapter) for bent or otherwise damaged pins. They are, after all, like 0.2mm wide with a 0.3mm spacing, so while they are protected inside a metal casing, they are quite delicate (the only, even smaller, connector I know of, is HDMI type D "Micro").
