crackling audio in Windows XP guest

[nbi]

Minor crackling in XP guest. This was introduced in 5.1.12 but is still present in 5.1.14. It is not as bad as the Windows 10 crackling, but is sufficiently annoying and did not exist prior to 5.1.12.

Host is Debian Stretch running 64bit kernel 4.9.2.

[Frank Mehnert]

I have removed superfluous comments. Ticket followers have them in their inbox.

This ticket interferes with #15189 (crackling audio with Windows (10) guests on Linux hosts) but I will keep it open because this ticket has the extra information that this problem apparently started recently with VBox 5.1.12 (5.1.10 was fine) and the guest is Windows XP which most likely uses AC97 while Windows 10 uses HDA.

nbi, please attach a VBox.log file of such a VM session, mostly to see your VM configuration (e.g. which host audio backend you are using).

[nbi]

Performed additional testing. Downgraded VirtualBox from 5.1.14 to 5.1.10. I was able to confirm that 5.1.10 is much better than 5.1.14 although some barely perceptible crackling is still present. I was pretty confident that an older kernel in conjunction with 5.1.10 did not produce any noise. Sure enough testing with kernel 4.1.17 did not produce any noise under 5.1.10. I am at a loss to explain this. Nothing in the linux software/hardware environment is different and the kernel build parameters are identical for these 2 kernels. I've reached the following conclusions:

1. In general there was an audio degrade/regression at some point after 5.1.10.
2. There also seems to be some form of kernel dependency which contributes to the problem (noise reduction from barely perceptible to none at all).

[nbi]

Problem present in 5.1.18.

[Frank Mehnert]

Many audio changes in the latest test builds. Could you try again?

[Frank Mehnert]

Please reopen if still relevant with VBox 5.1.24.