Is it permissible to work on a task to evaluate the response of a voice assistant, which includes playing and stopping songs, in addition to playing the Qur'an, knowing that this helps the voice assistant in playing the requested audio files?

There is no objection, in principle, to programming a website for voice command execution. It is like all other technologies that can be used for permissible or impermissible purposes, and the possibility of its use for forbidden acts does not necessitate prohibition.