Just notes because every forum post I came across for this was 10 years old, and all the references and links were out of date. Once you find the files, there's nothing difficult here.
From the Wiki "MrSID (pronounced Mister Sid) is an acronym that stands for multiresolution seamless image database. It is a file format (filename extension .sid) developed and patented by LizardTech (in October 2018 absorbed into Extensis) for encoding of georeferenced raster graphics, such as orthophotos."
Because they're patented, it was somewhat of a pain for me to find something to read them with. When searching, everywhere I found refereed files hosted on LizardTech's website. Because of the merger, none of these links still worked. I didn't want the installer or some huge software package, I littereally needed one binary. Finally found it here: https://www.extensis.com/support/developers/retired-sdks
I used the Linux 9.5.1 "Unified GCC 4.8 -- 64-bit" (http://bin.extensis.com/download/developer/MrSID_DSDK-184.108.40.20627-linux.x86-64.gcc48.tar.gz)
Download that and extract the contents. I'm on Windows using WSL and got some "Can not create symbolic link" warnings. Ignore these, or run 7z as admin. Browse into the \Raster_DSDK\bin directory and find "mrsiddecode". This is the file you need. You'll also need to apt-get install libtbb-dev and libgeos-dev.
$ sudo apt install libtbb-dev libgeos-dev $ ./mrsiddecode -i infile.sid -o outfile.jpg
Easy as that. Also works to output .png files (just change the extension) and I'm sure a host of other formats, but I got what I wanted.