What would be the best instructions for a slider that has no min/max value. For example, you are either all the way to the left or right of a question asked. You could also be in the middle, or anywhere within the slider. So the positioning of the thumb could be anywhere within the slider. Think decimal places. What is the best way to make this accessible and what output should the screen reader users get?
Is there really no min/max value, or is it that you don't want to distract users with numbers? The slider is very likely not infinite, so you probably want to use percentages as input for users who don't want or can't use the slider. You will need an extra number field that allows 0 to 100. It is preferably visible for everyone because not only screen reader users will benefit. (See also this excellent answer.) It can however be visually hidden and only accessible for screen readers (as described here).