Actually you CAN change the default sound. In your Alexa app go to the 3-line menu to the left of "Home", open "Reminders &i Alarms," select "Alarms" from the three selections offered (Reminders/Alarms/Timers), and click just below that on the words, "Manage alarm volume and default sound." From that drop down menu, you can select under "Custom Sounds," - Alarm. THAT side menu offers you these choices (all sounds that you can preview):
Adrift
Clangy
Clarity
Countertop
Focus
Glimmer
Nightstand (the true ringy alarm sound you'd probably like)
[more/others....]:
HSettings/You have a choice of, e.g.,