Audio slowed after update

[DFspx]

Hello,

after an update I have a problem with a guest system, and I would like to know if someone has some advice to solve it.

the original (working) configuration was MacOS 10.6 as guest in an older VB-Version (sorry, don't know which one) under MacOS 10.9 as host on a Mac Pro (end 2013 production). This configuration had no problems, everything worked.

A while ago I decided to update the host system to 10.11 (currently 10.11.6). As a direct result of the update the VB didn't launch at all due to its older version.
I then updated VirtualBox (to Version 5.0.20 r106931, currently running), this made the old guest installation usable again.

Unfortunately I later detectec that while the 10.6 guest is now mostly running correctly (including the emulation for older Mac programs which is the main reason for it), it now plays audiofiles slowed down to about half speed (estimate from hearing)

Does anyone know about such a problem, or in which part of the configuration the sound driver might gotten wrong data during the updates?
(I can't completely rule out that this might have happened after the update as it wasn't tested immediatly, but I consider a different cause unlikely, and it still has to be something in the VB audio configuration).

If you need more data or configuration screenshots or so, please ask (it might be a few days before I can answer however)