There should be audio with the HDMI connection. Is it possible to tap into this on the HUB board?
Yes, but the HDMI audio signal is digital (SPDIF). See the topic "sound on the pi-top". The signal is available on the hub connector and on the pi-topPROTO board. Raspian needs to be set up to provide hdmi audio (see above topic).
The pi-topSPEAKER converts that signal to analog. See "how does the speaker work" in