The main symptom: after a period of time (e.g. 0 to 30 minutes) using video or sound apps on the macOS virtual machine in a Mac equipped with a Presonus STUDIO audio interface, the sound (and video, if applicable) becomes choppy and stuttered. Sound and video on the host OS (Big Sur) continue to work fine. Background: I use Parallels to create a virtual machine (VM) running Mac OS X Server 10.6.8 (Snow Leopard) on my Mac Pro (2019) with a Presonus audio interface. My goal is to use old versions of software which will no longer run on later OS X versions. One specific application that I use is iMovie HD 6.0.4. This is where I first encountered the problem with stuttering audio and video. However, this problem is not confined to iMovie. It also occurs when playing back simple videos and even simple audio. For example, I can go into the Snow Leopard System Preferences->Sound->Sound Effects and playing a simple alert sound exhibits the choppy audio. Hardware: Mac Pro (2019), 16-core, 160 GB, AMD Radeon Pro 580X 8 GB PreSonus STUDIO 26c audio interface Software: macOS Big Sur all versions up to 11.2. However, problem first appeared before installing Big Sur. That is, I saw the problem with Catalina macOS 10.15.x. PreSonus UniversalControl 126.96.36.199835 Parallels Desktop 16 for Mac Version 16.1.2. However, problem first appeared in version released in late 2020 -- perhaps version 16.1.0. Mac OS X Server 10.6.8 (Snow Leopard) Parallels VM configuration: 4 processors (tried 2 processors with no luck) 8192 MB memory (tried other amounts with no luck) Hypervisor: Apple/Parallels -- no difference, adaptive on/off -- no difference. Note, however, that the Apple hypervisor appeared to help in earlier stages (i.e. earlier versions of Parallels) but it no longer helps. Graphics: 1 GB. Tried other sizes but they did not help. Sound->Sound Output: "Default" or "Studio 26c" exhibit the problem. (Big Sur Sound Output is set to "Studio 26c.") "Default" exhibits the problem. (Big Sur Sound Output is set to "External Headphones.") *** (Some Possible Big Clues here) ** If I set the Parallels VM Sound Output to External Headphones and use amplified speakers, it works fine (but this is not an acceptable solution). (Big Sur Sound Output set to "Studio 26c.") It appears that the Parallels VM has an incompatibility with the Presonus STUDIO audio drivers and/or Apple audio drivers.