Here's the command to play someplayer, then return to the desktop and play the song:
/usr/bin/someplayer & sleep 6 && wmctrl -k on && dbus-send --type=method_call --dest=ru.somebody.someplayer / ru.somebody.someplayer.toggle

sleep 6 is for 6 seconds, sometimes it's too short because someplayer start after 6 seconds depending on how many stuff you run on the n900, you can change it to 10 for exemple, make some tests!
Maybe you'll have to download wmctrl.
In terminal:
root
apt-get install wmctrl
I'll post the following later I have to go out

Edit:
dbus-send --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.set_extension_property string:volume variant:uint32:$((`dbus-send --print-reply --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.get_extension_property string:volume| tail -1 | awk '{print $3}'`-5))
for volume -5

dbus-send --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.set_extension_property string:volume variant:uint32:$((`dbus-send --print-reply --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.get_extension_property string:volume| tail -1 | awk '{print $3}'`+5))
For volume +5

dbus-send --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.set_extension_property string:volume variant:uint32:0
For volume mute

dbus-send --type=method_call --dest=com.nokia.mafw.renderer.Mafw-Gst-Renderer-Plugin.gstrenderer /com/nokia/mafw/renderer/gstrenderer com.nokia.mafw.extension.set_extension_property string:volume variant:uint32:22
for volume 10%

20%: change the last number to 26
30%: 30
40%: 35
50%: 42
60%: 52
70%: 62
80%: 72
90%: 82
100%: 100

I know the numbers are strange but when I simply put 10,20,30... 100 it was not linear...