How can I pass a variable to a page with a SEF url?

You have to either work it out with the WP_Rewrite class or add this to the .htaccess file

Options +FollowSymLinks
RewriteEngine On
RewriteRule ^sermon/(.*)/ /sermon?name=$1 [L]

UPDATE

Forgot to mention that the htaccess code is supposed to be above the wordpress generated htaccess code