Clicking and dragging the cursor between two points on the graph will cause Audacity to play only the selected section. Get comfortable with this, because highlighting particular sections of tracks plays a large role in editing and adding effects to audio files.
Basic Editing 101
Before you start tinkering with the nuts and bolts of an audio track, you'll have to make sure it's stopped and not playing. Not paused; completely stopped.
The Edit menu (and the Editing toolbar) contains base-level options like cutting, copying and pasting. Click Trim to remove everything except the selected section of the track. Choose Silence to replace the selected portion with--you guessed it--silence. Use Split to remove the selected portion, replace it with silence, and drop the selected portion into a new track of its own, which appears below the first track. Select Duplicate to copy (rather than remove) a highlighted section of audio and then drop it (as with Split) into a new track of its own.
The Effects menu is chock-full of options that augment or enhance the existing audio. You'll have to play with them all to get a feel for the vast array of tools available, some of which have names that don't describe their functions terribly well. Options such as Bass Boost, Echo, and Fade In are straightforward enough, but they have granular effects settings attached to them; Bass Boost, for example, lets you set specific frequency and decibel levels.
Be sure to back up your audio files before hacking them to bits in Audacity. If you're performing complex edits in a project, the Label tool comes in handy, especially if your tinkering extends over multiple sessions. To add a Label--which is basically the Audacity equivalent of a note in the margin--to a specific point in the file, click that point in the waveform graph and then select Tracks > Add Label at Selection. The label will appear beneath the graph, and you can type whatever description you want in it.
Finding the right recording settings
People frequently use Audacity to record podcasts or jam sessions, or to convert music from old cassette tapes and vinyl records into digital files. Recording audio from outside sources--such as a microphone, a guitar, a record player or a USB turntable--is fairly easy with Audacity, though fine-tuning the input sound may take some work.
Begin by plugging your input device into your computer (unless you're recording with a computer's internal microphone, of course). Audacity recognizes all kinds of inputs, including USB audio, standard mic-in, and the beefier line-in. That last tidbit is the secret sauce to recording old analog music sources, by the way; just connect your cassette or record player to your computer with a line-out cable.
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