Unfortunately, it seems that it's not possible to change the default reminder for calendar events in WP8 "Calendar".
You can find an entry about it in the feature suggestions website for WP, here.
An alternative would be to use another calendar application.
Unfortunately that is not possible with the Calendar on Windows Phone itself.
Two points to note:
1- Even if you have multiple Windows Live calendars configured, they are all grouped under "Reminders" and so you cannot even enable sounds for one calendar and disable them for another. However you can enable email reminders for particular calendars.
2- You ...