For physical sound interface(s) to be usable inside the container, you should expose them with --device /dev/snd/.
By default sndio will use rsnd/default device. This can be modified by supplying -f argument(s) as Docker container command.
/tmp/sndio: Contains sndio UNIX socket file.
The sndio daemon might want to adjust its process priority. Therefore it is required that the Docker container has SYS_NICE capability.
$ docker run -v sndio_socket:/tmp/sndio/ --cap-add SYS_NICE ghcr.io/kdockerfiles/sndio:1.8.1-4 -f rsnd/1