Speakers incorporating amplifier can be used for i-PRO cameras.
The common specification is as follows.
Audio output : φ3.5 mm stereo mini jack (monaural output)
Output impedance: Approx. 600 Ω (unbalanced)
Output level : -20 dBV
The specification of speaker which is acceptable for i-PRO cameras,
is available at the following URL.
Item:Spec Sheet (for each model of the camera)
Column: Input / Output - Audio Output