how to change a .wav to frequency >15000 Hz?


i have two soundtrack, and i want to mix them together into one like this:

soundtrack A is some human voice recording, i would like to make it to frequency >15000 Hz so that human ear can’t hear, but the track is still there, still playing…
soundtrack B is a normal music track.

what i want to do is to combine this two soundtrack, not to mute track A, both of them must exist, just track A we human ear can’t hear it.

is someone can help? this really mean a lot to me!!

Thank you very much!

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