I wish to report success in using a PCI to PCIe adapter card. Of necessity, I upgraded my Core Duo motherboard. The new one, an Asus Prime X390-p motherboard with an Intel I5 processor has no PCI slot so I expected that I would have to abandon my beloved M-audio Delta 1010 audio card. I knew that adapter cards existed but did not expect them to work for audio. Nevertheless, for an outlay of around £10, I bought one to try. I plugged it in and to my delight, it worked straight away. I have run it for a few hours so far, with no issues whatever.
Given that finding audio cards for Linux is, ‘A bit of a problem’, a whole new avenue of supply has opened; second hand PIC cards.