A method comprising: receiving input sound data (S DS) representing sound detected by one or more microphones of a first playback device (703, 103i); detecting, via a command-keyword engine in a first voice input represented in the input sound data, a first command keyword corresponding to one of a plurality of keywords supported by the command-keyword engine; after detecting the first command keyword, making a first determination, via a first local natural language unit, NLU (779, NLU1), employed by the first playback device, whether the input sound data includes at least one keyword within a first predetermined library of keywords from which the first NLU is configured to determine an intent of a given voice input; selecting, based at least in part on the input sound data, a second playback device (703, 103d) employing a second local NLU (779, NLU2) with a second predetermined library of keywords from which the second NLU is configured to determine an intent of a given voice input, wherein the second playback device is selected from among a plurality of additional playback devices of a media playback system, to transmit the sound data to, wherein each of the additional playback devices comprises a respective NLU configured to detect, in input sound data, keywords from a respective predetermined library of keywords different from the other respective predetermined libraries of keywords from which each respective NLU is configured to determine an intent of a given voice input; transmitting, via a network interface over a local area network to the second playback device of the media playback system, the input sound data; receiving, via the network interface from the second playback device, a response; and after receiving the response from the second playback device, performing an action based on an intent determined by at least one of the first NLU or the second NLU according to the one or more particular keywords in the voice input. receiving input sound data (S DS) representing sound detected by one or more microphones of a first playback device (703, 103i); detecting, via a command-keyword engine in a first voice input represented in the input sound data, a first command keyword corresponding to one of a plurality of keywords supported by the command-keyword engine; after detecting the first command keyword, making a first determination, via a first local natural language unit, NLU (779, NLU1), employed by the first playback device, whether the input sound data includes at least one keyword within a first predetermined library of keywords from which the first NLU is configured to determine an intent of a given voice input; selecting, based at least in part on the input sound data, a second playback device (703, 103d) employing a second local NLU (779, NLU2) with a second predetermined library of keywords from which the second NLU is configured to determine an intent of a given voice input, wherein the second playback device is selected from among a plurality of additional playback devices of a media playback system, to transmit the sound data to, wherein each of the additional playback devices comprises a respective NLU configured to detect, in input sound data, keywords from a respective predetermined library of keywords different from the other respective predetermined libraries of keywords from which each respective NLU is configured to determine an intent of a given voice input; transmitting, via a network interface over a local area network to the second playback device of the media playback system, the input sound data; receiving, via the network interface from the second playback device, a response; and after receiving the response from the second playback device, performing an action based on an intent determined by at least one of the first NLU or the second NLU according to the one or more particular keywords in the voice input.