When you purchase the right to perform a play you are given the dialogue, stage directions and music. Sometimes the script includes a description of the choreography and incidental sounds, and a summary of the set, costumes and props, but the details and the realization are up to you.
I have found that area microphones and shotgun microphones are not as effective as body microphones for community theatre. Only if I can get a microphone within an inch of the actor's mouth can I get enough gain before feedback to provide effective sound reinforcement. The problem with wireless body microphones is that you need one for everyone who sings or speaks, to make the voices consistent, and they are expensive. In most cases you can decrease the cost by sharing microphones between actors. For small plays this is easy to figure by hand, but for large plays juggling microphones gets complex. In my notes for each play I have recorded how I did microphone swapping.
Sound effects, even when not noted in the script, can add realism to a performance. When the scene is in a city park there should be a suitable background. Sometimes a sound effect can be a character, as when Noah talks to God in “Two by Two”, where God is represented to the audience as thunder. Sound effects can complete an action: a faked slap is much more realistic if the audience hears the hand hit the face. If a character listens to the radio you want a good radio show that sounds like a radio from the period. Telephone sounds also vary depending on the decade.
I believe in subtle sounds. Even if a sound is too faint for an audience member to remember after the performance, it can help to set a mood. In “Two by Two” the intermission takes place during the 40 days of rain. I put a boom box in the lobby playing rain during the intermission. For “The King and I” I played royal Thai music while the audience was entering the theater.
Generally I get sound effects for a show from a variety of sources. In community theatre I can often get friends to speak lines into my recorder. Often I record something I constructed, such as ice being crushed or a tire being thrown into some bushes. When I needed the sound of a music box playing “Have Yourself a Merry Little Christmas” I bought one from ebay and recorded it. Freesound.org is also a good source of sounds.
I use Audacity to edit and combine sounds, and convert them to 16-bit stereo WAV files at 44,100 samples per second. I use a Roland SP-404SX to play the sounds during a performance. It uses an SD card, so it is easy to transfer sound files from my computer to it, and it has large buttons for triggering sounds. With some practice I can time a sound to match the action of a stage slap. These devices are fairly expensive, even on ebay, so you may have to use a CD player. The Roland SP-404SX can play several sounds at the same time. If you are using a CD player and need to play a lightning strike to match the dialogue during a rainstorm, for example, you will need two CD players: one for the rainstorm and another for the lightning.