Product or service recommendation-type questions are, so far, okay to post on sound.SE.
That said, they still need to be of the variety that could be considered relevant to future visitors. They should ask for a product or service in the pursuit of solving a specific problem. Questions like, "what is the best condenser microphone" will likely not survive, while, "what should I look for in a condenser microphone for recording a cello on stage" would probably do quite well.
As far as the learning materials-type questions like the one you linked, so far, they have not become burdensome on the site. I could foresee, if their numbers got out of hand, having to come up with a specific policy on how to handle them, but so far, I think they're okay.
It would seem likely though, that there would only be so many answers to questions about where to start learning. I think it will probably become more likely to consider them duplicates of earlier, more in-depth Q&As. That may lend itself to a community wiki in the end.