Find Some Background Noise
To filter out the ambient background noise, you’ll need to find a section of audio where it can be heard alone, isolated. This might involve splitting a stereo track to two mono tracks (via the down-arrow on the Track Control Panel, and selecting Split Stereo Track).
Once you’ve found the audio clip select it with the mouse. Make sure the Selection tool is active, then left-click and drag the section of background, ambient audio. This audio selection will be used by the Noise Reduction feature in Audacity to find similar background ambient, and remove it from the track(s).
Use Noise Reduction in Audacity
With the audio selected (it will be highlighted), open the Effects > Noise Reduction feature. You’ll see a dialogue box with wizard. First, click the Get Noise Profile button. You have your profile selected already, so click this.
The Noise Reduction box will close. Then, select the portion of audio that you wish to remove the ambient sound from. If this is an entire audio track, left-click the track header to select all.
With your selection, return to Effects > Noise Reduction. In most cases, the default Noise Reduction, Sensitivity and Frequency Smoothing settings will work, so you can go ahead and click OK to accept these. It may take a while for the noise reduction to be applied.
You can preview your results. Hit File > Save to retain the changes.