The oscillator will swing between the transistor’s breakdown open and close voltages, which will result in a fairly high DC offset; see the oscilloscope picture here for an example:
(that specific build swings between 7.4 and 10.7 volts, which corresponds to a DC offset of almost exactly 9 V)
Depends on your mixing desk if that’s a problem. If you have an AC coupled input with ok headroom you can just plug it in as is. To filter, you can try anything from, say, 100 nF upwards. Line inputs are high impedance, so you shouldn’t need big capacitors.