Is it usually allowed to use grant money for voluntary contributions?

I have to guess on this one, but can imagine that they haven't mentioned it because they haven't thought of it. I also imagine that they would be somewhat upset to learn of it unless you make provision for it in the grant application.

The reason for my speculation is that few grantors are so rich that they don't much care how their, perhaps, limited funds are spent.

But if you use something that is explicitly funded by donations, then you may be able to make a case that you should donate. Your suggestion of Patreon's involvement suggest that you may be in this situation.

But not all "donations" are prohibited or discouraged. Putting grant funded software in public repositories and making it open source or giving generous licenses to it is often encouraged. Another possible form of donation is the funding of a person so that they can donate their time and efforts to worthy public projects that can't afford their own funding.

I would ask the grant administrator before doing this, and suspect that different people will give different answers. And if the donations are a minuscule part of the grant, it is less likely to be forbidden. But that, too, is speculation.

The answer to this question will vary significantly based on the laws of your country, policies of your institution, and your contract with the grant funder. Although your profile specifies that you are not in the United States, some readers may find value in an answer specific to U.S. law and practice. I recognize this won't directly apply to you.

Mea culpa aside, for U.S. federal grants under Uniform Guidance the answer is definitely "no". To be an allowable cost, the expense must:

Be necessary and reasonable for the performance of the Federal award and be allocable thereto under these principles. (2 CFR § 200.403)

Since the video content would have been available to you even without a donation, the donation is not necessary or reasonable for your grant-funded project.

For projects that are non-federal, or otherwise not under Uniform Guidance, the details will likely be found in your specific contract with the funder. Research institutions typically have sponsored projects offices with trained staff who are able to assist you with answering these kinds of questions.

I'd argue that you're not funding them for the work they've already done, but to produce more of the lectures you found useful. So, why not enter into a conversation with the person making the talks directly about this? Ask them if there are upcoming talks they'd like to make, and then see if you can help fund the making of those.

This would then be very similar to, for example, paying for changes to a piece of software (even if those changes are then freely released)*, and should be possible to get past finance/the funding body.

You probably can't use grant money to pay for the existing ones though.

(*I'm a developer in UK academia, and paying open source developers for the changes we need is something we've had approved before, and it's simply been billed as "software development" on the grant)