A short single beep is pretty much the de facto standard for beep codes indicating a successful POST (power on self test). Back in the day when every case included a system speaker, that little reassuring beep was standard.
Then a few motherboard makers started including little piezoelectric "button" speakers on a few of their boards so case makers took that cost cutting opportunity to stop including them with cases. But sadly, integrated motherboard system speakers never became a standard and never became widespread.
So still to this day, only a few motherboards include them.
HOWEVER, the ATX Form Factor standard still requires all motherboards support system speakers so the motherboard
front panel I/O header (where the leads connect for the case power button, power LED, reset button, drive indicator light, etc.) still provide a connection if there is no integrated speaker. In fact, some boards have an integrated speaker and provide support to connect another via that header. Note the 4 pins in the upper right of that white header connection are for the speaker. It is a 4 pin connector though there are only two wires going to the speaker.
On all our builds here, if a motherboard does not have an integrated system speaker, we always include one. That little beep is reassuring for us, and the clients seem to appreciate it too. If your system does not have one, it is very inexpensive to add a
System Speaker. Note that price is for 20 speakers!