1] Come on now, simply translate what you are reading. You have quoted many ma’amarei chazal in the coffee room already. You seem smart enough to translate:
Hayom-Today
Yom Rishon- The first day
Bashabbos- of the week
Shebo- That on it
Hayu- They were
Halevi’im- The Levi’im
Omrim- Said
Shir- Song
B’bais Hamikdash- In the Mikdash
2] It’s a mishnah which reviews the weekly portions of shira.
3] Everyone has their own thoughts. Some think about whether the line at the Dunkin Donuts drive through window will be too long, some think can’t the chazzan speed up a little bit, some imagine the levi’im singing in the mikdash and begin to harmonize with them, some are busy wrapping up their tefillin and have a foot out the door. You get the point. Whatever floats your boat!