Flash not supported on webapps
For Youtube, search for "Browser" in Dash and open https://www.youtube.com/html5 then press "Request for HTML5 player".
For Grooveshark the only way is to change link "http://grooveshark.com/" with "http://html5.grooveshark.com/" in grooveshark.user.js file located in "/usr/share/unity-webapps/userscripts/unity-webapps-grooveshark"
P.S. I don't really understand why do you need to use webapp, but most of the functions are not gonna work anyway, until webapps will be fixed (not particular webapp, but webapp system).