If I use a CC-BY background music track in my video on YouTube, is it enough to give credits to the musicians in the video description or must credits be shown in the video?
Section 3(a)(2) of the CC BY 4.0 license describes how to satisfy the attribution requirement:
You may satisfy the conditions in Section 3(a)(1) in any reasonable manner based on the medium, means, and context in which You Share the Licensed Material. For example, it may be reasonable to satisfy the conditions by providing a URI or hyperlink to a resource that includes the required information.
It is definitely reasonable to put hyperlinks to the original track into the YouTube video description, and it would be less reasonable to show a link in the video itself.
However, a link to the original work is not the only attribution that may be required under CC BY 4.0. In particular, you may also want to identify the original track (e.g. by name), the artist's name, and that the track is CC licensed. It may very well be quite reasonable to show all of that in the video itself, for example with an overlay at the bottom of the screen when the music starts playing, or in the closing credits.
My rule of thumb: wherever you assert your own authorship, you should also provide attribution for other's contributions that are part of your work.
This principle was also codified in the earlier license version, CC BY 3.0 Section 4(b):
The credit required by this Section 4 (b) may be implemented in any reasonable manner; provided, however, that in the case of a Adaptation or Collection, at a minimum such credit will appear, if a credit for all contributing authors of the Adaptation or Collection appears, then as part of these credits and in a manner at least as prominent as the credits for the other contributing authors.