According to WAI-ARIA, aria-valuenow is required for role=slider, even if it has aria-valuetext. From NVDA, JAWS and VO testing it looks like aria-valuetext works well as an alternative. While slider is intended as a value input mechanic, it is often used in video players. Their time value is better expressed with text than it is with a number. Having a number there at all may be more confusing than helpful.
According to WAI-ARIA, aria-valuenow is required for role=slider, even if it has aria-valuetext. From NVDA, JAWS and VO testing it looks like aria-valuetext works well as an alternative. While slider is intended as a value input mechanic, it is often used in video players. Their time value is better expressed with text than it is with a number. Having a number there at all may be more confusing than helpful.