2) There is a TB add-on for Windows for this but the Linux version does not have
this capability. You can install a utility such as AllTray from the repos. There
are several others that allow you to minimise to the tray, as well, but it isn't
automatic. You need to open AllTray and then click on the window of the program
you want to minimise. A bit of a pain, but it works.

1) My sounds work in both KDE and Gnome. Check your preferences in TB (under
General).

Why does one want to minimise to the tray? [not questioning the advice
but i'd like to understand why].
When i've run TB i have just minimised the window and/or in ubu just
switched to another desktop. I have always set the email client to
retrieve email automatically [Edit--Account settings--Server
settings--{check box}Check for new mail every 15 mins].
How is what i do different to minimising to a tray [other than a
little icon as opposed to a minimised window - aren't they basically
the same]?

You make a good point. I don't use this option. However, Windows users are used
to minimizing to the system tray and having things stay resident in memory
because real estate is limited due to having only one desktop. I suspect it is a
carry over from time spent in Windows. Just a guess.

With Alltray you can minimize any window to the system tray, such as Firefox.
However as you point out its usefulness is dubious. With the virtual desktops in
Linux, you can just keep them running and switch desktops, which is what I do. I
don't think it saves any RAM if they are minimized.

I only use a second desktop occasionally... what
one does with four i don't know! Useful if one is doing programming
and keeps different things open i suppose.

i noticed that thunderbird delete the mails from my original mailbox after
consulting, consequently i can't re-read my emails in the mail box!! how can i
overcome this!

Click on Edit, Account settings, Local folders, and disk space. Uncheck
the box Alway delete read messages.

Not sure what you mean by "original mailbox". If you are talking
about the server mailbox, then you can choose to leave messages
on there by, in Thunderbird, going to Edit>Account Settings.
On the left, select Server Settings. Check the box that says
"Leave messages on server". You can choose whether to just
leave them on for a set number of days or until you go in to
the mailbox and delete them.