After using Rockbox for some time I found out about some very cool features. It has so called crossfeed. This features sort of simulates the experience of listening to speakers when using headphones, by delaying and filtering part of the signal from one side and routing it to the other side. I had to have it for my laptop too. So I searched to web, without much successs. However, I found out that there is a very interesting plug-in architecture for linux (LADSPA) which is also natively supported by ALSA. So I set on to develop a little plugin. This can now be used to create a virtual sound card in ALSA that applies this headphone transformation. You can find the source and a sample ALSA-configuration here.