Sounds like: One long two short then a failure buzz-beep.
Typically a Video error beep code but I would check your PSU to make sure it is providing proper volts before doing anything else -> assuming it all was working properly and re-try the original parts again.

I don't suppose your cmos battery is dying? If you unplug it from power for a little while do you lose your bios settings?

Horun wrote on 2023-12-05, 02:50:

Sounds like: One long two short then a failure buzz-beep.
Typically a Video error beep code but I would check your PSU to make sure it is providing proper volts before doing anything else -> assuming it all was working properly and re-try the original parts again.

I tried multiple ISA and PCI graphics cards, Trident 8900C, 9000i, GD5422, S3 Trio 64V+, etc., and nothing changed.

Regarding the PSU, I also replaced it and can confirm that the PSU works fine on other motherboards.

NHVintage wrote on 2023-12-05, 03:11:

I don't suppose your cmos battery is dying? If you unplug it from power for a little while do you lose your bios settings?

I tried removing the CR2032 battery and nothing changed, black screen and alarm and that's it.