Record ambient (background) sound effects, such as the noise of a crowd, city or forest, separately from action and dialog. Alternatively, choose from a library of recorded sound clips, available online or on CD. Plug-ins to your digital audio workstation (DAW) may come with free tracks, or allow access to other sound libraries of free clips. Layer these sounds into the recording at appropriate points to help establish general mood. For example, birdsong conjures rural tranquility; the hiss and suck of waves over pebbles suggests the menace of a storm.
Add "hard" sound effects such as shots, screams, fire crackle or footsteps. Manipulate pre-recorded sounds electronically or use "Foley" special effects techniques live in the studio, such as asking someone to blow a whistle, scratch glass, crumple chip bags or rattle keys in time with the filmed actions.
Add music to suit the background mood or the characters' feelings. Choose martial band music, for example, to convey a mood of national pride; or romantic orchestral music for a passionate love affair. Match unrequited teenage love with tender, wistful ballads in a minor key; or hard-hitting, fast-paced gangland feuds with heavy metal.
Edit with extreme care. Synchronize sound to action with absolute precision. Adjust tone and dynamics (loud and soft) so that the sound effects sound realistic and the music supports the mood unobtrusively. Use the DAW's graphic display of sound waves to check you are assessing tone, pitch and volume objectively.
Liaise continually with the director (if any), who has the final say on which shots to include and whether your audio input makes them more or less effective.