Explanation: When receiving a Single Sideband (SSB) signal, your receiver's Beat Frequency Oscillator (BFO) must be set to precisely the correct frequency relative to the incoming signal's suppressed carrier to properly demodulate the voice. If the transmitting station's frequency is off (either too high or too low) relative to where your receiver is tuned, the BFO will beat against a shifted signal, resulting in audio that is either too high-pitched, too low-pitched, or garbled and barely readable. Since "all other signals sound normal," it indicates the problem lies with that specific signal's transmission, not your receiver's general functionality. A defective receiver synthesizer, VCO, or BFO circuit would likely affect *all* SSB signals, not just one. Therefore, the most likely cause is the abnormal signal's transmitter operating off its intended frequency.