Now I'm writing my review of a product, and I'm not sure if I use would, it sounds politer.
I've just used the speaker, but I wouldn't say it's the best.
If I use “would” here like the above, does it sound softer than "I don't say it's the best"?
More than politeness, that "wouldn't" indicates tentativeness. Perhaps you are certain that the speaker is not the best, but you don't want to sound so categorical. Then, you use what we call an understatement.
Instead of saying:
you can say:
Replacing "wouldn't" with "don't" sounds really awkward. It would really help to get more context about the sort of feeling that you're trying to communicate though. Right now it sort of sounds like you wouldn't say that it's the best speaker, but there are some things about it that you liked and would like to list later on.