Remy was using the 32-bit version of USBEHCD.SYS which is the USB 2.0 host controller driver.
And as far as I understood the readme, this version of USBEHCD.SYS does not support isochronous transfer. But the one from the 11.17 stack (16-bit) did.
Or has this now changed for the 32-bit version of USBEHCD.SYS ? If that is the case, then it will indeed be sufficient to just use my version of USBAUDIO.SYS / USBAUD2.SYS together with the AN stack.