405 lines from Raspberry Pi 5

[Nick]

Now can you run a USB DVB-T stick and play that out on the composite?

There is no audio out on the Pi 5. I think the easiest solution to that is a USB sound card.

You can use a USB sound card, like the UGREEN one, or an I2S audio hat from IQaudIO (now owned by RPi) e.g. their DAC+ or DigiAMP+, or similar devices from others.