The Tivo remote only has codes for tv's and Audio device's volume. The Roku Soundbar is too new, anyways the commands preset to the volume keys on the roku remote work with all rokus, not just the soundbar, so it not strictly an audio device commands.

When any roku sees the roku volume command it initiates a hdmi-cec signal sent over the hdmi cable to the tv its connected with.

So the tivo remote can not control the roku soundbar directly, but it can control your tv, then the tv will control the roku soundbar via hdmi-cec.