Hello patriciafromprescot,
Apple Support Communities is a great place for help with the
alarms on your iPhone X. You mentioned the volume of the alert decreases. Does
this continue to happen if you change to a different sound? Also, have you
tried the steps outlined in the section titled If the alarm volume is too low
or quiet in the following article?
How to set and change alarms on your iPhone
If not, complete the steps outlined there as they may help with
resolving this issue. If the issue continues, the next steps would be restarting
your iPhone and updating iOS if needed. Those steps can be found in the
following articles.
Restart your iPhone
Update your iPhone, iPad, or iPod touch
Let us know how that goes!