Under what condition are sound levels produced by a plant not usually a problem?

The condition under which sound levels produced by a plant are not usually a problem is when the indoor sound levels of nearby residents aren't negatively affected. This means that the plant's operations do not create sound levels that intrude upon the quiet and comfort of surrounding homes or businesses. If residents are not disturbed by noise, then the sound produced by the plant can be deemed acceptable within that community context.

The other choices, while relevant in the conversation about sound management, do not directly address the impact on nearby residents in the same way. Monitoring sound levels is important for compliance and safety, but it does not necessarily mean that the sound is acceptable to residents unless it's within a certain threshold. Wearing hearing protection is crucial for workers within the plant to safeguard their hearing but does not address the external impact of the plant's noise. Meeting sound guidelines is also important; however, guidelines are generally established based on population and environment, so it's the direct effect on residents that remains the key factor in this context.