General Volume Controls
The volume for both the headphones (listening) and microphone (recording) can be controled in Audaicy®. This can be done using the two sliders shown in the image. There may also be an option to select the input device.
Track Specific Controls
There is a volume control slider on the audio track itself below the mute and solo buttons. The volume control slider is labeled with a - and +. If you move the slider towards the + you will increase the output volume. You will not see any changes in the track itself, but when the track is saved, it will be saved at a higher volume than before.
Another way to amplify a track or just part of a track is to use the Amplify function. Select the portion of the track you want to amplify and select the menu item Effect -> Amplify. A small dialog will appear asking you how much you want things to be amplified. There is both a slider and numeric controls for the amount of the amplification in decibles (dB).
A default level of amplication (Amplification (dB)) may be suggested. That amplification is the greatest increase possible without clipping the audio. Clipping happens when the sound becomes too loud and starts distorting. Clipping will occur if the volume is increase beyond a certain amount. Audacity protects you by requiring that you specifically allow clipping to occur.
You can also change the New Peak Amplitude (dB). This value will set a peak amplitude of a certain number of decibels. A value of zero is the maxium allowed before clipping. Numbers above 0 will cause clipping. Numbers below 0 will just be quieter.
To improve the quality of amplified audio, you may also want to normalize the volume of your track or tracks. This will set the maximum volume of all selected audio to the same peak level. This can be useful if you have recorded one part of the track at a different volume, and need to make both parts sound equally loud. To normalize tracks in Audacity, go to Effect -> Normalize and change the settings to get the desired result. Checking the box labeled "Remove any DC offset" will senter the waveform on the 0.0 amplitude level. This will allow you to raise the volume of the entire clip later. The number in the box labeled "Normalize maximum aplitude to:" shows the maximum amplitude you would like the selection to have.
Audacity's Noise Removal effect allows you to remove certain types of noise from your audio sample. This allows you to cut down on background "hum" or buzzes or clicks in the background, resulting in a cleaner final product.
Audacity's noise removal is a two-step process. First, select the type of noise you wish to remove (such as the sound of a fan in the background) by highlighting it in the timeline. It's best to select a sample which includes no speaking or any sound you want to keep. Basically, the selected sample should be an "empty" section of the recording. Then go to Effect -> Noise Removal and click "Get Profile". This tells Audacity to analyze the selected sample so it knows what to filter out. This profile can be re-set at any time, allowing you to remove several types of noise from the same project.
To actually use the noise profile generated, select the entire track or just the section from which you wish to remove the noise, and then return to the menu item Effect -> Noise Removal. This time go to the second step and click the "Remove noise" button. The process may take several seconds depending on the length of the track. If you do not get your desired effect the first time, you can repeat the noise removal process, or use Edit -> Undo to undo the noise removal and try again.