|Start up beeps do not need Windows sound, a small squeaker (my jargon) is fitted on the motherboard.|
Two things can happen in this situation, mis-plugging or something not firmly pushed home. Another is that the slightest movement can cause an edge connector to move onto oxide despite it being connecting perfectly for years. Clean edge connectors of RAM, circuit cards, SATA power and signal connectors, with a soft pencil eraser. Plug the cleaned items in and out a few times to clear any oxide off the sockets before pressing them firmly home.
Always pop back and let us know the outcome - thanks