This is what I found so far.
The DS1802 is a I2C stereo log digital pot, but hard to get and costs 15 euros.
The PT2258 is a 3 channel stereo mixer and is very cheap (1 to 2 euros), 5 to 10V.
The AD5206 is hard to get.
The LM1971 is a single channel attenuator, costs about 7 euros.
If I use the PT2258, I need two of them for a mixer for 4 stereo channels.
The I2C address is selectable, so they can be used together on the same I2C-bus.
Is the chip any good ? will it work at 5V ?