Toshiba Stereo Music Centre SM-D30,Turn Table, SX-N330 Speakers

Use a string to wrap around the table and pulley to get close approximation of size. Reduce number by a smidge and get that.

the best (and easiest) way to download your phonebook is to use computer or external sd card

Sounds like the belts on the tape player have perished

There are a ton of places online for finding the replacement belts. Just put in your model number followed by "replacement belt". The first thing you need to do is remove the rubber pad to reveal the platter (the spinning part you put the record on). If you cannot just lift the platter at this point then you will have to remove the record pin in the center. This, in most cases, pulls right out. You may have a C spring holding the platter down at this point which you can get off with a small flat head screwdriver. Keep a finger on the spring or hold it down with something as you release it. If you don't it will fly 30 ft. Once you have the platter off you will be able to see where the belt goes. I don't know your model specifically but the drive motor will be in the center or off to one side. Either way it should be fairly obvious on how to attach. If not feel free to respond to this again and I will hel you out.

Music only plays through those two speakers, only movies with 5.1 format will play through all your speakers

All you need do is measure the diameter of the cassette belts and put them in google. If say it's 53mm go for a 50mm belt.
With the turntable one you will also have to add the width.
When you take them out make a note of how they fit.

you need a phono pre-amp or a mixer...

There are a few settings that will just give you a stereo output. The first one is with the Pro Logic II. If you press the PLII Mode button on the remote it will go through a series of settings from MUSIC ➞ CINEMA ➞ PRO LOGIC ➞ MATRIX ➞STEREO ➞ MUSIC. Pro Logic setting will only produce sound from the front speakers and Stereo from the front speakers and the sub. Put it on Matrix for 5.1 surround sound.
Also for speaker settings follow the instructions below.
Signal outputs and frequency responses from the speakers will automatically be adjusted according to your speaker configuration and whether certain speakers are used or not.
In Stop mode, press the MENU button.
Press the Cursor down button to move to Audio and then press the ENTER button.
In the SPEAKER SETUP, press the ENTER button again.
Press the Cursor button to move to the desired speaker and then press the ENTER button.
For C, SL, and SR, each time the button is pressed, the mode switches alternately as follows : SMALL ➞ NONE.
For L and R, the mode is set to SMALL.
SMALL : Select this when using the speakers.
NONE : Select this when no speaker is connected.
Press the RETURN button to return to the previous level.
Press the EXIT button to exit the setup screen.
It also seems that with this system, if you are on 5.1 surround but your music source is only in stereo, you will only hear stereo.

we need a belt for a Sansui turntable sr-2050c.

Is there a switch to turn on the headphone port? Is there a control on the headphones to adjust the volume (on a volume level on the side of the laptop)? Also, when the headphones are plugged into the laptop, go into your sound settings and turn up the master volume (and wave volume if it is low.. If that works, make sure to turn it back down when disconnecting the headphones).

Also, do the headphones work in another device (stereo, etc..)?