If it doesn't beep then it's not the monitor - it's not even getting thru the POST. What PSU is in the system? Do you have another PSU to try in the system? Were there any power surges prior to his PC not working?
<font color=red>You're a boil on the arse of progress - don't make me squeeze you!</font color=red>
Well if you took the computer to a less reputable Pc repair shop they could have fouled up somewhere installing the motherboard, and processor. Also ram sticks wouldnt do that unless all were gone. My friend had one bad one and his computer ran fine.
I'm assuming that, when turned on, you're getting fans spinning and whatnot. I'm also assuming that your speaker header wires are plugged into the mainboard properly. If either of these assumptions are wrong, let me know...
If the system is getting juice but not getting to the point where it can offer you BIOS error codes (blips and beeps), chances are it is either the CPU or the motherboard. It sounds as if the system can't even get to a point where it can indicate what hardware is causing problems. Provided you're system is getting power, I'd bet that your CPU is cooked or, second, your mainboard is shot.
Take it back to the shop and have them stick another P4 chip in it. Don't leave until you see your system alive and running.