Select SEC_TO_TIME(SUM(TIME_TO_SEC(timediff(timeOut, timeIn)))) AS totalhours
FROM volHours
WHERE username = 'skolcz'
Si no, entonces tal vez:
Select SEC_TO_TIME(SELECT SUM(TIME_TO_SEC(timediff(timeOut, timeIn)))
FROM volHours
WHERE username = 'skolcz') as totalhours