To use the audio trigger you only need a separate/special hardware in case your smartphone/tablet switches off the internal microphone when a jack is plugged to the audio output. This results in a permanent current value of "0" shown by the audio trigger. Should this value still change with ambient noise level, when the IR dongle or cable is plugged in, you don't need further hardware to use the audio trigger.
If your smartphone/tablet switches off the internal microphone, you will have to provide an external one. This can be done by an adapter as shown in the schematic below. To make live easy, such adapters are commercially availabe and are sold under the heading "music adapter", "headset adapter", "stereo adapter" etc. It is important that on one side the adapter shows a 4-pin audio jack (3.5 mm) and a 3-pin audio socket (3.5 mm) with integrated microphone on the other side (see pictures below).
A "Music Adapter iPhone" (2,90 € incl. shipping) with a 4-pin audio jack and integrated microphone (interestingly, the two audio channels are swapped, but this does not realy matter for our purposes).