The iMac occasionally boots without the Apple bong

This iMac has some issue where it does not always boot with the Apple bong after power is removed if I need to remove the power cord. This is the second time it has done it and I was initially able to clear it up with a PRAM reset (no SMC needed at the time). However, it has now come back for a second time. I have a 20” ALS 20/2GHz/1GB/250GB/SUPER/56K/BT/AP EMC 2056 , if it helps.

So far, here is what I tried:

• PRAM reset (~3/24/19)
• SMC reset (4/5/19)

I checked the CMOS battery (Panasonic CR2032) and it reads 3.07V, so it was likely replaced before I got it or it’s an original cell that still works. I can usually clear it up by adjusting the volume and that solves the issue, but it may eventually come back - even with an SMC reset performed.

Should it come back, what should I look for? I have a QP538 machine, so I’m firmly out of capacitor plague territory. I don’t think it’s the caps because of how out of the range mine is. The software was a recent install as well, since I had to start fresh due to how I got the machine; I also wanted Classic support, so it required a 10.4 downgrade.