What?
For an overview of the new features, have a look at theRenoise 3.1 Release Notes please.
In summary, Renoise 3.1, being a 0.1 update, continues where 3.0 left off and now brings the features that got introduced with Renoise Redux into Renoise as well. That is, various enhancements to the sound-engine and DSP devices, a whole big new set of filter DSPs everywhere, over-sampled & band-limited sample playback options and more.
Thus Renoise now also finally is compatible with instruments that got/get created with Redux and also shares Redux’ XRNL sound library format and content. This means that any kind of user presets (instruments, phrases, samples, multi-samples, modulation-sets, DSP chains and DSP FX presets) which you’ve created in Redux will show up in Renoise too and vice versa, when both are installed on the same machine.
Installed sound libraries will now also show up in both, Renoise and Redux. During the beta there will be no factory instrument content in Renoise apart from a few tutorial instruments, but you can download Redux’ Sound Library packs and install them for Renoise. See “Extras” page at the Renoise Backstage.
Apart from the Redux compatibility and feature-set, we tried to get a few other bigger and smaller “Renoise only” features too: Biggest other feature is the ability to route MIDI outputs from VST & AU plugins into other instruments, but also a better handling of DSP FX chains in Renoise’s sampler with the ability to route chains to specific tracks.
3.1 is available to all registered users and free for everyone who registered Renoise at version 2.1 or later.
Where?
The beta can be downloaded at the Renoise backstage at:
https://backstage.renoise.com
Public demo version of 3.1 will be available shortly before it goes gold.
How?
There are now also two new sub-forums in this forum which deal with the beta testing. Please post anything related to 3.1 there, so we can hear you.
Thanks for helping us to test-drive the next version of Renoise!
Enjoy Renoise 
wishes