A single beep from the system can be almost anything, or a warning from Windows itself. Try to think of EXACTLY what you were doing immediately before that beep happened. Did you click on an inactive window, did the system stall for a little bit? It may seem odd, but this is critical to note.
Issues with RAM more commonly come as a system stability problem (BSOD, random errors).
For example, undervolted & improper clocked RAM will not MemTest or function correctly ... but is perfectly fine. It took a while to properly volt and time my system (OCZ RipJaws on a Gigabyte P55A-UD3). Once i got the timing right, the memory was fine.
This was true back in the day when OCZ stuck to 2.1v memory in DDR2 DIMMs (this is my actual system from back in the day): http://www.youtube.com/watch?v=zjeGmrAmYoc
Once I volted to 1.6v on each dimm, the system was back to normal.