The speed of sound varies with air temperature, which is what the linked graph shows.

Technically the speed of sound does vary with density, but as you change altitude there's also a change in pressure which exactly cancels that out. In the end only temperature and gas composition alter the speed of sound.

As long as you're inside the plane (and hopefully it's not 217 K or -70 °F, per the graph) then the speed of sound should be unchanged.