this library doesn't provide neither easy nor copyright-safe usage about sso

[SumianVoice]

This is all true. Furthermore, according to the license, "appropriate credit" is defined as:

If supplied, you must provide the name of the creator and attribution parties, a copyright notice, 
a license notice, a disclaimer notice, and a link to the material. CC licenses prior to Version 4.0 
also require you to provide the title of the material if supplied, and may have other slight differences. 

That means you'd need a license file to be distributed with your music, but that can easily get removed or lost, which means you need to put it in the meta tags. That means a copyright notice bigger than the one in this actual project's license file.

Containing Sonatina Symphonic Orchestra <https://github.com/peastman/sso> which is distributed
under the terms of the Creative Commons Sampling Plus 1.0 license
<https://creativecommons.org/licenses/sampling+/1.0/> 

Problem is, there is no copyright notice in the license. The copyright holder didn't add a "Copyright John Smith 2011", so do we just assume it's Mattias Westlund 2011? What about the other contributors to the project since then? Missing those and using any of their contributions or modifications would be a copyright infringement without attributing them.

There is actually no legally safe way to use it whatsoever, and no matter how you distribute your music, you can be sued by anyone who acts on Mattias Westlund or the other contributors' behalf. I don't think you can be sued by random third parties, since the license says nothing about that I believe (as in, unlike the GPL iirc) but your idea of heirs etc is correct. You can basically be extorted.

There is also of course the prohibition on advertising clause which means that if you have a bandcamp or a youtube or etc, and you have music on it that uses SSO and music that doesn't, you're advertising one with the other, and are in breach of the license.

There is this:

e. Attribution and Notice
[...] You must keep intact all copyright notices for the Work and give the Original Author credit 
reasonable to the medium or means You are utilizing by [...]

It could be argued in court that "reasonable" does not cover a license whose conditions cannot even be met, but who wants that hanging over them?

I think a warning should be added to the readme.

[peastman]

I don't think that's actually the effect of the license. It doesn't cover the samples (which came from other sources as described in the README), only the sfz scripts used to control them. Attribution is only needed for derived works that remix the scripts, which is to say, other sample libraries derived from this one that reuse the scripts. Music created with this library doesn't contain any part of the scripts, so the license doesn't restrict it.

I agree it's still not the best choice of license, but I don't think it's that much of a problem.

[SumianVoice]

Unfortunately the license info at the bottom of the readme is not clear enough to constitute actual free use, so you'd have to track down the actual original person who recorded or paid for the recordings to happen and find where they gave or didn't give permission. In the meantime I'm just using the samples directly from UIEMS. (I only just realised the samples actually came from here, I switched to UIEMS when I found out about the license issue without knowing that's where it came from originally haha...)
They are not cropped or modified, just raw .aiff files, so they need editing to be useful in music production, but at least they gave explicit permission: "...may be downloaded and used for any projects, without restrictions"

I found out later that indeed I was confused when I read the legal code, since they mix "works", "derivative works" and "collective works" together.

Edit:
To clarify though, the edited UIEMS samples are now under the CC license, NOT the license given by UIEMS, so if you use the SSO version of the UIEMS samples, you're still having to use them under the CC license, so be aware of that. Technically, you would have to edit the originals yourself to not be under that license.