Routines now allow skills to be invoked, with this skill you can set a default sound or stream via a URL and then use it in a routine. Play a funny noise, play an alert, a stream or anything else you can think of! Once set up, activate the skill via any routine using triggers like time of the day (useful as an alarm), motion sensors or Alexa Buttons.
Enable the skill, link your account and say "Alexa, open Routine Stream" to start setup. Once you have run setup you can change your default URL at will by going to https://routinestream.com.