Use built in MIXER command. But all this should be in the README file. Too bad that some people don't read it. 🙁

1MIXER
2  Makes DOSBox display its current volume settings.
3  Here's how you can change them:
4
5  mixer channel left:right [/NOSHOW] [/LISTMIDI]
6
7  channel
8     Can be one of the following: MASTER, DISNEY, SPKR, GUS, SB, FM [, CDAUDIO].
9     CDAUDIO is only available if a CD-ROM interface with volume control is
10     enabled (CD image, ioctl_dx).
11
12  left:right
13     The volume levels in percentages. If you put a D in front it will be
14     in decibel (Example: mixer gus d-10).

ok i resolve ... win7 has a mixer that permit to adjust volume of single program.

best regards!

Weird solution but glad you got it working... For proper volume management type MIXER SB 50:50 and press enter in DOSBox. I hope that makes sense, it's the same thing as the DOSBox documentation mentions. Keep in mind that the built in Windows mixer adjusts the volume on every channel. That includes PC speaker, CD Audio and everything that produces sound. I find the built in mixer much more useful for fine tuning stuff if you find something too loud or quiet.