Massively compress bitrate from mp3 files software for Windows 7?

I´m looking for a freeware or otherwise fully functional software to compress multiple mp3 files in a folder, if possible including subfolders as well. I've found something but working for individual files, which makes the task near impossible. I want to accomplish this reducing the bitrate from 256k to 64k, due very limitated space issues. I know that for today standards it seems rare, but there are many files, and in these circumstances it would make a difference.
Any suggestions? Thanks in advance.

InspireSoft

I successfully used "mp3 Quality Modifier" from above link maybe 6 months ago.

It's a free download.

It gives you a lot of control on the bit rate, stereo, mono, vbr, etc. Bit rates down to 10 I think.

My original mp3s average around 4.3 MB in size, with an average bit rate of about 210.

As a test, I used a bit rate setting of 65-105 vbr mono and the resulting files were about 25% the size of the originals.

Using 65-105, vbr joint stereo, the resulting files were about 33% the size of the originals.

ignatzatsonic: Excellent, something like this is what I was looking for. You really made me save lots of time and frustrations. It worked.