There are WiiM compatible wireless speakers (Audio Pro A10 and C10 WiiM edition, Edifier speakers with the Linkplay module) but I don't know if any are battery powered.
Simple is a bunch of Amazon Echo speakers but playback of local files can be an issue.
Erin tested a fully assembled pair. They cost $3630- $3805 per pair plus shipping (~$80 to Mobile, AL) according to the GR Research website.
https://gr-research.com/product/nx-studio-pair/
Pure speculation on my part, Danny sent Erin the $3805 per pair speakers for testing.
Someone needs to figure out a way to disguise this speaker and send it to Danny to be measured and improved. He will say that the top end is too hot and modify the crossover to make it flat on axis.
According to Perplexity.ai WiiM supports:
MP3
AAC
ALAC (Apple Lossless)
APE
FLAC
WAV
WMA (Windows Media Audio) lossy but not WMA lossless.
OGG
AIFF2
The answer to the other question that one should ask:
FAT32
NTFS
EXT4
Here's a list of some of the choices/competition out there:
https://www.audiocircle.com/index.php?topic=191348.msg2002037#msg2002037
The Moondrop discdream 2 is a top loader.
I am impressed with Zorin OS running on a spare old Dell laptop with an 8th gen I5 processor. If I didn't need to use Microsoft Office I would likely run it instead of Windows 11 on my primary machine.