How to convert .xlsx to .txt?
Another way is rename it as .zip and unzip it as all the .***x files are just zipped folders containing xml. Inside you will find a folder "xl" with a subfolder "worksheets", inside is an xml file for each worksheet. The format of them is pretty simple and should be easy to parse with any of the xml packages.
The ssconvert tool that comes with Gnumeric can convert xlsx files to text:
ssconvert Book1.xlsx file.csv