Which of the following is a common sign of an electrical short in vehicles?

A blown fuse is a common sign of an electrical short in vehicles because fuses are designed to protect the electrical circuits by breaking the connection when the current flow exceeds safe levels. When a short occurs, it causes an excess current flow that can lead to a fuse blowing, as the fuse melts to prevent damage to the wiring and components in the circuit. This is an effective way to safeguard the vehicle's electrical system from more extensive damage caused by a short.

The other options do not directly indicate an electrical short. Muffled sounds from speakers might relate to audio system issues or speaker malfunction rather than a short circuit. Poor fuel economy usually indicates issues with the engine, air-fuel mixture, or other mechanical components, rather than being a direct consequence of electrical shorts. An overheating engine can result from cooling system failures, cooling fluid issues, or mechanical problems, rather than a direct link to an electrical short. Thus, blown fuses stand out as a clear and relevant symptom of this specific issue in the vehicle's electrical system.