Probably the most accepted view of sola scriptura, among those who defend it anyway is:
1. The Bible is the sole infallible source for the Christian church.That is not the ONLY view of sola scriptura! I would agree, that those who have studied sola scriptura that they would agree with this definition - but there are others who take things a bit further.
There are some who hold to:
2. If it's not in the Bible, it does not have to be believed.Now that's not too far off from the first definition I posted - but takes it a bit further. If it's not in Scripture, don't believe it.
Then there are some who hold to:
3. If it's not in the Bible it is not true.Well, there are many inherent problems with that statement. Literally speaking, 2+2=4 is not in the Bible, but it is not only true, it is infallibly true in a base 10 numeric system.
Now let's look at a variation on the previous definitions:
4. The Bible is the sole source for the Christian Church.This one leaves out "infallible" from definition 1 and thus becomes like definitions 2 and 3.
Another avenue which some adherents to sola scriptura take is:
5. Sola scriptura is an axiom for Christians - and does not need to be found in Scripture to be true.The tact here is they do not have to defend the lack of scriptural support for sola scriptura - because it is an axiom - a self evident truth which requires no proof. Those who use this argument are or have already essentially conceded that sola scriptura is not scriptural - and thus have abandoned any attempt to prove it scripturally, resorting to a very dogmatic stance of it being axiomatic.
While Catholics do not hold to sola scriptura - what IS their view on Scripture and the teaching of sola scriptura?
A. The Scriptures are a source of infallible knowledge from God Almighty. The Scriptures are the Word of God, but they are not the only infallible source of knowledge/teaching in the Christian Church.
Of course, Catholics accept the Magisterium of the Catholic Church as a source of infallible teaching, and included in the Magisterium is the Pope himself - when he speaks ex cathedra.
B. The teaching of sola scriptura is not scriptural!There is no passage in Scripture which teaches sola scriptura! There are many which teach satis scriptura, which would be the (material) sufficiency of Scripture (but not the formal sufficiency), but again, nowhere does Scripture teach that a book (e.g. the Bible) is or ever would be the sole infallible source of truth for the Christian Church. This view demonstrates how sola scriptura is a self-refuting proposition - especially for those who hold to definitions 2, 3 and 4 above! For those holding to one of those definitions, sola scriptura should be rejected - for it is not found IN Scripture! Again we must stress, satis scriptura is not the same as sola scriptura! To make the claim that Scripture is "enough" or "sufficient" is not the same claim as saying it is the "only" or "sole" source of truth for the Christian. Back to the point, since no passage of Scripture teaches sola scriptura - then the concept itself is extra scriptura (Latin for outside of Scripture).
Excerpts from debates between James White and Gerry Matatics and James White and Patrick Madrid:
Protestantism and Authority, a video from Fr. Barron: