In [this post](https://forums.androidcentral.com/showthread.php?t=194842&p=2111288&viewfull=1#post2111288) at Android Central, the suggestion is:

1. Open a root explorer, go to `/system/media/audio/ui`
2. Rename `PowerOn.ogg → PowerOn.ogg.disabled`, `PowerOff.ogg → PowerOff.ogg.disabled`

This requires root.

<br>

<sup>This seems promising, but in my HTC E9 phone (Android 5) these files don't exist :(</sup>