A method for inserting a first audio signal into a bitstream which comprises a downmix signal and associated bitstream metadata; wherein the downmix signal and associated bitstream metadata are indicative of an audio program comprising a plurality of spatially diverse audio signals (110, 120); wherein the downmix signal comprises at least one audio channel; wherein the bitstream metadata comprises upmix metadata for reproducing the plurality of spatially diverse audio signals (110, 120) from the at least one audio channel; wherein the method comprises - mixing the first audio signal with the downmix signal to generate a modified downmix signal comprising at least one modified audio channel; - modifying the bitstream metadata to generate modified bitstream metadata; and - generating an output bitstream comprising the modified downmix signal and the associated modified bitstream metadata; wherein the modified downmix signal and associated modified bitstream metadata are indicative of a modified audio program comprising a plurality of modified spatially diverse audio signals (113, 123), wherein - the plurality of spatially diverse audio signals (110, 120) comprises a plurality of audio objects (110, 120); - the plurality of modified spatially diverse audio signals (113, 123) comprises a plurality of modified audio objects (113, 123); - the bitstream metadata comprises object metadata for the plurality of audio objects (110, 120); - the downmix signal and the modified downmix signal are reproducible within a downmix reproduction environment; characterised in that: - the object metadata of an audio object (110, 120) is indicative of a position of the audio object (110, 120) within a 3-dimensional reproduction environment; and - modifying the bitstream metadata comprises modifying the object metadata to yield modified object metadata of the modified bitstream metadata, such that the modified object metadata of a modified audio object (113, 123) is indicative of a position of the modified audio object (113, 123) within the downmix reproduction environment. - mixing the first audio signal with the downmix signal to generate a modified downmix signal comprising at least one modified audio channel; - modifying the bitstream metadata to generate modified bitstream metadata; and - generating an output bitstream comprising the modified downmix signal and the associated modified bitstream metadata; wherein the modified downmix signal and associated modified bitstream metadata are indicative of a modified audio program comprising a plurality of modified spatially diverse audio signals (113, 123), wherein - the plurality of spatially diverse audio signals (110, 120) comprises a plurality of audio objects (110, 120); - the plurality of modified spatially diverse audio signals (113, 123) comprises a plurality of modified audio objects (113, 123); - the bitstream metadata comprises object metadata for the plurality of audio objects (110, 120); - the downmix signal and the modified downmix signal are reproducible within a downmix reproduction environment; - the object metadata of an audio object (110, 120) is indicative of a position of the audio object (110, 120) within a 3-dimensional reproduction environment; and - modifying the bitstream metadata comprises modifying the object metadata to yield modified object metadata of the modified bitstream metadata, such that the modified object metadata of a modified audio object (113, 123) is indicative of a position of the modified audio object (113, 123) within the downmix reproduction environment.