How to implement Qbasic's Sound and Play Statement which can let the PC speaker sound some music?

Those were implemented using a BIOS interrupt (between 0x10 and 0x21).

Do a search for Programable Interval Timer (PIT), it is the thing that makes the tones.

I remember there was a port.... forty-something hex, which controled the frequency of the speaker, then another one lets you enable or disable it... but it's been a long time that I don't check that.
Anyway, when you want to develop a PLAY routine, you must do something more complex: remember that musical notes do NOT grow lineally with frequency. Suppose note 0 is LA440, then:

frequency = 440*(root(12;2)^note)

That is, if the frequency of a note is F and you want to get the frequency of a note which is one semitone higher, you must multiply F by the twelveth root of two. The same way, to get the frequency of that note, but in the next octavia, you simply multiply F by 2.

Hope this is useful to you.

Thank you! I know this, what you said is Equal Temperament, there are already many programs.
But I would prefer Just Scale(harmonic tuning), that's why I want to do it by myself.

see more info at :
http://www.phy.mtu.edu/~suits/scales.html