A sound signal receiving and decoding method to be performed by a terminal apparatus connected to a first communication line and a second communication line with a lower priority level than the first communication line, the method being characterized by comprising: a reception step of, for a frame in which a difference in frame number between a monaural code included in a first code string input from the first communication line and an extended code with the closest frame number to that of the monaural code among extended codes included in a second code string input from the second communication line is less than a predetermined value, outputting the monaural code included in the first code string input from the first communication line and the extended code with the closest frame number to that of the monaural code among the extended codes included in the second code string input from the second communication line, and for a frame in which the difference is not less than the predetermined value, outputting the monaural code included in the first code string input from the first communication line; and a decoding step of, for a frame in which the difference is less than the predetermined value, obtaining and outputting decoded digital sound signals of C, C being an integer that is at least 2, channels based on the monaural code output in the reception step and the extended code output in the reception step, and for a frame in which the difference is not less than the predetermined value, outputting decoded digital sound signals obtained based on the monaural code output in the reception step as the decoded digital sound signals of C channels, and the extended code is a code indicating a feature parameter which is a parameter that indicates a feature of a difference between the input digital sound signals of C channels and has a low temporal resolution. a reception step of, for a frame in which a difference in frame number between a monaural code included in a first code string input from the first communication line and an extended code with the closest frame number to that of the monaural code among extended codes included in a second code string input from the second communication line is less than a predetermined value, outputting the monaural code included in the first code string input from the first communication line and the extended code with the closest frame number to that of the monaural code among the extended codes included in the second code string input from the second communication line, and for a frame in which the difference is not less than the predetermined value, outputting the monaural code included in the first code string input from the first communication line; and a decoding step of, for a frame in which the difference is less than the predetermined value, obtaining and outputting decoded digital sound signals of C, C being an integer that is at least 2, channels based on the monaural code output in the reception step and the extended code output in the reception step, and for a frame in which the difference is not less than the predetermined value, outputting decoded digital sound signals obtained based on the monaural code output in the reception step as the decoded digital sound signals of C channels, and the extended code is a code indicating a feature parameter which is a parameter that indicates a feature of a difference between the input digital sound signals of C channels and has a low temporal resolution.