I have an RPi with minimal Raspbian that's connected to a speaker, and have set up the sshd. How can I redirect all audio from my main machine, which runs Arch Linux, through the RPi to the speaker?
You need to configure Raspberry Pi as an audio streaming server and the Arch Linux as an audio streaming client. In this case, both Raspberry Pi and the Arch Linux have to be joined to a local area network.
By this link, you can do it over TCP to a server running the PulseAudio daemon. Follow procedure up to "Zeroconf (Avahi) publishing".