Thanks for a quick reply. I'll follow that for future installations.

Yesterday I downloaded missing dependencies from http://maemo.org/packages/* and installed them using dpkg -i, after that I was able to install Extra Decoders Support package and it works for me now.

Note: missing dependencies was:
1. libwildmidi0_0.2.2-2maemo1_armel.deb
2. gstreamer0.10-plugins-bad-extra_0.10.14-git265-0maemo10+0m5_armel.deb