Adjusting system volume using OF

hey, ... and how can I 'insert' a variable into this appleScript?
I mean, on this case, what if I want to convert an int to string to use just a line for every volume changing values?
I tried something like this:

std::string systemAppleScript; //all the script
std::string systemVolumeValueStr; //just the string for the variable value

systemVolumeValueStr = ofToString(systemVolumeValue); //systemVolumeValue is the changing value
systemAppleScript = "osascript -e \"set Volume " + systemVolumeValueStr + "\"";
cout << "systemAppleScript: " << systemAppleScript << endl;

system("systemAppleScript");

Obviously this code doesn't works because system is not receiving the content of the string. This is the console output in Xcode:

systemVolumeValue: -10.1
systemAppleScript: osascript -e "set Volume -10.1"
sh: systemAppleScript: command not found