go to "usr/share/sounds/deepin/stereo/" in your file manager then right click and click open in terminal then type "sudo su" hit enter then it will ask for your password type it in and hit enter. After that type "chmod 777 sys-login.ogg" hit enter that will unlock the file then type "rm sys-login.ogg" and enter which will remove the file; that will make it so there will be no start up sound. if you want to turn it on and off make a copy of the file before you do this and then when you want to turn it back on just put it back.

I know this looks weird but i was looking around for a while to find an solution for this and there was no complete answers. so when I finally figured this out for my self I decided to make make it easier for others to do this.

My suggestion is lower your speaker