I would answer this question by looking at art in general. Art is the expression of all that is human experience. Some art is beautiful and inspiring and some is dark and depressing. Both ends of the spectrum, and everything in between, represent human experience for better or worse. The reality is that horrible things happen. Art will express this reality whether or not it is authentically sourced. If you want to include the sound of an ambulance siren in your piece of art, you are using it to express an experience (or to convey the essence of that experience).
Ethically speaking, there are a few things to consider, as you mentioned in your question. If the siren sample came from a real life experience, did you fulfill your civic obligation to make sure that everyone is safe and/or receiving the needed care? If so, I don't think that there are any ethical issues with using the sample in your art. I, and probably many others, would actually argue that it is more authentic and holds more meaning, and therefore does a better job of expressing the experience, i.e., more artful.
On the other hand, if you did not fulfill your civic obligation, perhaps to get a better recording, then the ethics get sticky. If you have a legitimate reason for not helping, like knowing that the ambulance is close and that there is little you could do, then I see the use of the sample as morally sound. If you watched a man die instead of saving him... not so much. If you can justify not helping, then I see it as morally useable.
Either way, this is still viably art and its ethical implications are for the artist to bear. Let's say you didn't help out, then feel awful about it. Your expression of that will be creatively driven by not only the experience itself but the ensuing guilt, which is an extension of that experience. The resultant art could be more creatively inspired and better convey the essence of the experience. This may be a 'better' piece of art but the artist may come to loathe its beauty.
You also need to consider the audience. If the man dies and the art you are making is for a commemorative video for the family... not good. If the sample is for a documentary about preventing suicide (assuming that's what the man did) to be viewed by a large audience, it is questionable but I think that it would provide the realism to get the audience invested in the serious subject matter.
Intention is another factor. If the art intends to make fun of something terrible, the morality is entirely subjective. In the art of Comedy, for another medium, jokes about pedophilia and rape are surprisingly prevalent but I am fairly certain that almost everybody, including the pedophiles and rapists themselves, would say that they are terrible things. These jokes still exist and I hear people, that I know have been victims, make and laugh at these jokes, while others find it offensive, including non-victims.
If your intention is to display the reality of a terrible thing, potentially to expose the injustice, then I see this as ethically sound. I think of the Pulitzer prize winning picture of the girl burned by napalm in the Vietnam War. This photo is horrific. It's right up there with the Holocaust photos. But it has this tangible quality that brings a morbid sense of beauty, like capturing a photo of a rare animal; you hear about it and know that it exists but seeing the picture makes it real.
Another thought to consider: What if it is generally agreed upon that an artist's choice to use a specific sample was ethically unsound? Would a third party using that sample have the same ethical implications? It would run through a similar ethical filter to the original but certain implications cannot be passed on to the replicator, such as not helping at the scene of an accident to get a better recording. These things sort of get 'stored' in the sample, making the replication harder to be seen as ethically sound but still easier than the original.
Ultimately, we are talking Ethics, perhaps the most subjective topic, next only to Emotion. The artist, other individuals, their society and the world at large will all have a different take on the ethical implications. In the current technological state we can readily have access to sample libraries and synthesized sounds; memes are constantly spread across the internet with different variations; videos stream at our desire. This expansion of media and increase in replication and variation bring a sort of anonymity to sample use and, with it, decreased ethical implications. In short, over saturation has decreased the likelihood that someone will realize the sample is authentically derived and the ethical implications are largely the artist's to bear.