I assume that as you want a scart to phono cable your TV does not have phono audio outputs on the rear. If it does then just use them, but if not make sure that at least one of the scarts is wired to output the audio, as many TV's will have input only connections on scart.
If you do have a scart output on the TV then you can use a lead such as
This and just leave the composite video phono unconnected. Or a cheaper option would be a scart adapter such as
This and any spare stereo phono lead you have.
With any scart to phono connection make sure the scart is either switchable or is wired in the correct direction. This type of cable is directional and if you get it wrong you will get no signal to the amp.
Mark.