small power interruption

The battery will keep the sound going if you do encounter a momentary interruption, but it won't help to keep the loco going over any bad contacts or dirty track.  The main purpose of the battery is for use in conventional control, when you don't really want the sound to stop and restart every time you change direction or stop for a moment. But it does come in handy if you get a short power interruption when using command control. 

No!  it is only for a small power interruption to keep sounds from cutting out.  I do not put batteries in any of my lionel locos.

I don't have the 6-11210, but I've just looked at the manual on-line, and I would think that the sound card is in tender, together with the volume control, and that the wired tether is purely the connection to the speaker. However, I could well be wrong, and it might be that the sound card is in the locomotive, and the volume control operates via the IR tether. In this case, the wired tether could be passing battery power through to the locomotive sound card. Perhaps somebody more knowledgable of this locomotive can help further here?

I also noticed that the manual does recommend fitting the battery for all uses, with one stated intention being to prevent the sound shutting down if you do get a momentary power interruption.  I fit batteries into all of my Legacy units for this reason.