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You can use Standard Additions' command set volume. Since AppleScript 1.9.4 the syntax is the following:
set volume output volume 75 --> number from 0 to 100
You can also specify "input volume", "alert volume", or mute the sound using the "output muted" parameter.
Previously, the syntax was "set volume 5" (from 0 to 7).
And, in past operative systems, you can use Jon's Commands' set sound volume to command.