Do laptops have a POST beeper?

It depends on the laptop whether it has an internal device to Beep on error cod.

If it does, it will show under Hidden Devices in Device Manager. It can be disabled there as well.

My laptops (two) do not have internal devices and so use the built-in speakers. No room inside for any extra things.

Other laptops may have internal speakers which can be disabled.

The article below illustrates how to find the speakers and disable them if you have them.

Laptop internal speaker

If you’re running Microsoft Windows, you can disable the internal speaker or PC speaker by following the steps below.

Open the Device Manager

In Device Manager, click View and select Show hidden devices. Once
done, Non plug and play Drivers becomes visible in Device Manager.
Expand Non plug and play Drivers by clicking the plus and double-click

In the Beep properties window, click the Driver tab. On the Driver
tab, if you want to temporarily disable this device, click the Stop
button. If you want to permanently disable this device, under the
Startup type, select Disabled.

Any laptop I have used has regular speakers and I think these are the only ones you can use to play any kind of music.