The default beep is controlled by a driver under, c:\windows\system32\drivers\beep.sys. Maybe if you switched it out with a driver from XP/Vista it would increase the volume.
To see this driver you have to open device manager then click on View > Show hidden devices. Then under Non-Plug and Play Drivers you’ll see “Beep.” This is the driver that runs your beep. Right click on it then choose properties and go to the driver tab, then click on Driver Details… This shows the file version which looks like it’s been change with Windows 7. So replace it with an older version and see what happens.
Procedure to follow
1. Boot from an UBCD4WIN disc (or BartPE if you don’t have an SATA hard drive)
2. Take ownership of the “C:\Windows\System32\Drivers\Beep.sys” file and give the local “Administrators” Group “Full Control” permissions.
3. Rename “Beep.sys” to “Beep.old” (just in case)
4. Copy “Beep.sys” from an XP machine to this location.