Unfortunately timer delays such as this stopped working on all google nest devices weeks ago, possibly after the recent firmware update. It has been reported hundreds of times, many threads about it, and Google say they are aware and a fix is awaited in the future at some date.
The nearest thing that can be done is not a solution so not perfect but a kind of temporary workaround as follows.
Create a Routine on the Google Home app with a single action command of 'Stop Playing'. Additionally create a starter on the routine for when you want the routine to action of a set time of day, e.g. for instance at 23:00 or of course whatever time would be more suitable. Then the music or sleep sounds will stop when the routine activates at that set time every day or night. Not perfect but the best you can do until this very annoying bug is fixed.
