Is the Holy Qur'an merely the source of belief? What other sources should we consult to understand the teachings of the faith?
The entire religion is housed in the Holy Qur'an and the Sunnah. The Holy Qur'an reforms both our belief and actions. Therefore, many religious teachings have been explicated in the Holy Qur'an. The Sunnah consists of the practices which the Almighty Allah has taught to us through the Holy Prophet (sws). Your question is directed to the people who take the Holy Qur'an as the only source of religion. The religious teachings reached us through the Sunnah have been referred to by the Holy Qur'an as known and conventional practices so it does not explicate them. However, if something of the sunnah has not been presented in the Sunnah then the Holy Qur'an has clearly explicated its methodology. A prominent example of this is the method of making ablution.
(Translated into English by Abid Mahmood)