ofxStk, Synthesis ToolKit addon

to amplify the signal just multiply each sample by the same value

for (int i = 0; i < bufferSize ; i++) {
  output[2*i] = amplitude * frames(i,0);
  output[2*i+1] = amplitude * frames(i,1);
}

If you want to have a slider with Decibels then to get the amplitude I believe the you do

amplitude = 10.0^(dbValue/10.0)