the video and photo apps allow deletion, but not the music one yet.
you can still delete songs when browsing the device as a network drive (wi-fi file sharing / usb file sharing).
in windows, start->run-> "\\220.127.116.11\media", where 18.104.22.168 is the IP of your playbook (see settings->about->network). username is playbook, no password by default. ensure that file sharing / wi-fi file sharing is enabled in settings->storage & sharing.
On a mac, from the finder Go->Connect To Server->smb://22.214.171.124/media, where 126.96.36.199 is the IP of your playbook.