How will mp3PRO content play on mp3 players?

The mp3PRO technology is a dual-rate system. The backwards compatible mp3 audio bit stream is run at half the sampling rate than the enhancement, thus to be able to generate mp3PRO bit streams the digital audio input data must be sampled at 32 kHz or above (44.1 or 48 kHz recommended).

That means:

mp3PRO content played on an mp3PRO decoder will have sampling frequencies of 32 kHz, 44.1 kHz or 48 kHz.

The same mp3PRO content played on an mp3 decoder will result in sampling frequencies of 16 kHz, 22.05 kHz or 24 kHz.

The number of channels will not change whether such mp3PRO content is played on an mp3PRO system or on an mp3 system, in case of "mono" and "stereo" content.

In addition to the "mono" and "stereo" formats, the mp3PRO format adds the so called Low-Complexity Stereo ("LC Stereo") technique to the plain mp3 format:

Mono mp3 content can be enhanced not only by applying the usual PRO enhancement for higher audio bandwidth, creating monoaural mp3PRO content, but also by LC stereo to make it a stereo channel mp3PRO bit stream.

This feature is designed to allow stereo coding even at extremely low bit rates. A plain mp3 decoder however would play LC stereo enhanced mp3PRO bit streams as mono.

That means (for LC stereo encoded mp3PRO content):

LC stereo encoded mp3PRO content will play as "stereo" on an mp3PRO system (at sampling frequencies of 32 kHz, 44.1 kHz or 48 kHz).

The same LC stereo encoded mp3PRO content will play as "mono" on an mp3 system (at sampling frequencies of 16 kHz, 22.05 kHz or 24 kHz).