A method for inserting a first audio signal into a bitstream which comprises a downmix signal and associated bitstream metadata is described. The downmix signal and associated bitstream metadata are indicative of an audio program comprising a plurality of spatially diverse audio signals. The downmix signal comprises at least one audio channel and the bitstream metadata comprise upmix metadata for reproducing the plurality of spatially diverse audio signals from the at least one channel. The method comprises mixing the first audio signal with the at least one audio channel to generate a modified downmix signal. The method further comprises generating an output bitstream comprising the modified downmix signal and the associated modified bitstream metadata indicative of a modified audio program comprising a plurality of modified spatially diverse audio signals.