Great work on the PC-X ... and good to see Axel "on board".
A few finds ... I had wrongly assumed that the PC-X used the OMTI 5300 MFM hard disk controller (which has a connector for a tape streamer as opposed to the OMTI 5100) ... Axel's PC-X uses a DTC520B. Now I suddenly wasn't sure if I had imagined mention of the 5300, but it's there ... in the Servicehandbuch Teil 1, page 217 relative to the PDF.https://dl.dropboxusercontent.com/u/55419307/Siemens%20PC-D%20Servicehandbuecher.zip
I'm still baffled by the beep after the "Test End" message, which should be quite short.
Servicehandbuch Teil 1, page 194 says that the loudspeaker is driven directly by Timer 1 of the 80816 (if I read the source right, that's implemented), and that "to help switching on and off of the speaker, one bit of the Device Control Register is used ... Bit D5 = 0 switches the speaker off, Bit D5 = 1 switches it on"
There's also a sort of beeper on the keyboard that can produce key clicks and beeps (duh!) ... cf. page 174.