Yes, you can play it on your phone with the screen off. You're still playing the video and processing the graphical data.

What would not be permissible is stripping the audio track from the video to play separately. That would generally be copyright infringement and there are limited circumstances under which a fair use defense would apply.

To be clear, if you turn off a device's display when it's playing a video, it usually stops decoding the video stream from the container, because it knows the destination video framebuffer isn't visible. It's doing the same thing you're talking about—"stripping the audio track from the video to play separately"—just without writing down the result.

You can get the same effect without turning off your screen if you tell e.g. VLC to stream only the audio from a network video stream. All VLC will do, in that case, is to request only the audio segments of the stream, and not request any of the video segments.

Consider this another way: is a web browser infringing a website's copyright when it makes a "derivative work" of the page by blocking the ads on it?

Or, hopefully even clearer: is a CD player infringing the CD's copyright when it applies an EQ preset to the audio as it plays it?

There's a very fine point I'm trying to gesture at here—the point where something goes from being "the creation of an infringing derivative work" to "the way that a device which reproduces the work for the purpose of consumption, chooses to do that reproduction."

To be clear, you're still processing the same file. You're just not processing all of it for the sake of efficiency. If you want to get into the very technical details, then it depends very heavily on what type of codec you're using since some codecs interleave video and audio data, while modern codecs generally don't (and in fact require separate audio and video codecs).

Consider this another way: is a web browser infringing a website's copyright when it makes a "derivative work" of the page by blocking the ads on it?

Yes, under copyright law, since the article and the page are separate copyrightable works. The question is does a fair use exception apply? (For something like reader mode, the answer is likely yes, but it gets more complicated for an ad-blocker that just removes ads while retaining other elements of the page.)

Or, hopefully even clearer: is a CD player infringing the CD's copyright when it applies an EQ preset to the audio as it plays it?

No, the CD player isn't a person and isn't infringing anything. The user of the CD player is creating a derivative work by applying an EQ. An analogous case (several actually) discusses this point with respect to running computer software. Courts ruled that the copy was necessary for a user to "enjoy" (make use of) their license. And with respect to the EQ, a fair use exception would apply. And even if it didn't, then a compulsory license would apply (in the US at least), and you would owe the song creator a fraction of a fraction of a penny for every performance.

There's a very fine point I'm trying to gesture at here—the point where something goes from being "the creation of an infringing derivative work" to "the way that a device which reproduces the work for the purpose of consumption, chooses to do that reproduction."

No, it's actually a very bright line. Either you're working with the original, or you're working with a derivative work. The question is whether your use of the derivative work you've created is covered by the license, or by fair use.

> What would not be permissible is stripping the audio track from the video to play separately

Even for personal use, where you aren't sharing the extracted audio? I am pretty sure that would be allowed.

Arguably it could be fair use but strictly speaking it's copyright infringement.

It's generally not worth the copyright owner's resources to pursue something like that unless it's happening on a much larger scale.

Fair use is not infringement.