How to stop bleeping sound?

http://support.apple.com/kb/HT1547

1 beep = no RAM installed

2 beeps = incompatible RAM types

3 beeps = no good banks

4 beeps = no good boot images in the boot ROM (and/or bad sys config block)

5 beeps = processor is not usable

Read the fist thread in the More Like This section at the bottom left of this site.

What are the random bleeping sounds on my computer?

This has been the case since first using it when purchased in 2010. How do I turn this noise off?

You waited 5 years to ask? Why didn't you contact Apple while under warranty?

Startup tests have a particular number of beeps to indicate RAM errors…

Power On Self-Test Beep Definition - Part 2 - Apple Support

Yours sounds like something else. Possibly software - but that seems unlikely if the volume is muted.

It could be the hard disk or some other failing components - do you have a full bootable backup or a Time Machine backup? I'd strongly recommend you make one before attempting too many tests, in case it stops working. Do you have any disks inside the CD/DVD drive?

Isolate it from all non-Apple devices, connect only the mouse & keyboard, reboot. Does it still beep?

Can you record the beeping with a microphone or mobile phone etc? Is it louder at one side or in any other position?

Reboot into safe mode.

OS X: What is Safe Boot, Safe Mode? - Apple Support

See if the beeps still happen. Safe mode will disable many system extensions, so do not be surprised if it is slow or certain things do not work, this is normal. Test a s much as possible. Reboot to go bad to normal.

I would normally suggest you take it to an Apple store, however 5 years is an awful long time to wait to address an issue like this, you will be out of warranty, even if you had Applecare.