Audio not heard from guest OS while it isn't in foreground

[akin]

Host OS: Windows 7 64-bit Ultimate Edition (US-English) [All official patches/updates from Windows Update are installed)]
Guest OS: Windows XP Professional SP3 32-bit (US-English) [All official patches/updates from Windows Update are installed)]
Guest OS application: Avaya IP SoftPhone R 6.0 SP7 (6.01.93)

Guest audio is set to AC'97. Appears to work fine for a while after booting up guest OS. When host OS is then in foreground, and guest is in the background or minimized, etc. sound continues to be heard from the guest OS for a time. Then after some time, audio is only heard from the guest OS when it is in the foreground. When it isn't, no sound is heard from it.

This is problematic as I rely upon the VM for 32-bit VoIP software for work-related use, which isn't 64-bit compatible and I need to be able to hear the VoIP phone ringing in the guest OS while working in the host OS. There are no error messages or indications and all apps, etc. appear to be working fine.

The guest OS presently has 2 GB RAM dedicated to it. The host OS has 4 GB total (so 2 GB guest, 2 GB host).

-Power management is disabled in the guest OS (no screensaved, no sleep mode, etc.) it is set to "Always on." Suspend/hibernate is disabled.

-The CPU is an Intel Core 2 Quad 2.66 GHz (Q6600).
-Intel G33 motherboard
-All drivers installed on the host OS are the latest release available (video, motherboard / chipset, audio, mouse, etc.)

Thank you.

[vbox4me2]

You'd have to play around with the GA versions, some older ones seem to work a bit better then 3.1.2, also when the Host has focus the Host might grab the audio device back for its own use.