A decoder (115) generates a multi channel audio signal, such as a surround sound signal, from a received first signal. The multi-channel signal comprises a second set of audio channels and the first signal comprises a first set of audio channels. The decoder (115) comprises a receiver (401) which receives the first signal. The receiver (401) is coupled to an estimate processor (405) which generates estimated parametric data for the second set of audio channels in response to characteristics of the first set of audio channels. The estimated parametric data relates characteristics of the second set of audio channels to characteristics of the first set of audio channels. The decoder (115) furthermore comprises a spatial audio decoder (403) which decodes the first signal in response to the estimated parametric data to generate the multi-channel signal comprising the second set of channels. The invention allows use of spatial audio decoding with signals that are not encoded by a spatial audio encoder.