When characters exert effort, they show it in their speech. As he says the plant line, he suggests he's trying to get it free at that very moment by showing the effort in his voice. Generally, this means adding some well-placed grunting. We can convince listeners someone is across the room without walking away from the microphone. The actor simply leans away or to the side of the microphone to sound fifteen to thirty feet away.
Sound effects are another misunderstood aspect of radio dramatics.
They are far less necessary than most novices realize. It's not enough to hear a crashing sound. Then we hear the crashing sound. Your students can do whole plays without complicated sound effects. Experiment with simplicity.