The switch you are thinking of only has three contacts in a row.
The type of switch I am talking about has two rows of three.
If you wire your sound reactive mod through the first type of switch both the left
and right triggers would be tied together, then if you press the right switch both the
left and right would light up at the same time.
If that is ok with you then you can use a single row switch.
If you want each switch to operate independent of the other you need
a two row switch so they are separate in the push button mode.
With the switch slid one way the lm386 drives both the leds to the
beat of the music. With the switch slid the other way, the button
press makes the corresponding led to light up.
In the top part of the picture the switch has two rows of contacts
This is called a double pole switch.
The double row keeps the left and right trigger leds operating separately
so when the right switch is pressed only the right led goes on.
If you wire them to a single row switch ( Single Pole ) then the two
leds will interact because they are connected to each other.
So when you press the right switch both the left and the right go on.
Just depends on how you want it to work