The ways are endless. At the end of the stage play, the characters may not say very much but they appear to be either dead or ecstatic and the audience, able to tell which, leaves the theatre satisfied. It determines whether your listeners believe in the world you have created. The ambience is determined by sound effects, the writing, the music, everything. A good radio play is good drama, full of conflict and even action, evoked through dialogue, music and sound effects, in that order.
Sound effects are an important ingredient, but by far, the least important.
Sound effects merely sketch in the action or punctuate dialogue and don't generally provide background in the way sets or locations do in plays and films. Sound effects suggest action, but they can easily be confusing or misinterpreted if relied upon too heavily. Intrusive sound effects will make the dialogue harder to follow. Just the same, sound effects can turn a mere script into a real-time drama as opposed to just a story being told. Sound effects add realism, but should be used sparingly.