Repurposing Cellphones as Bluetooth Transmitters?

One use I haven’t yet seen for disused phones (especially Androids) is as bluetooth transmitters, using them to send input from other devices to headphones or speakers.

iPhones can accept sound input via their charging ports. (Do Android phones have this aux-in capability via their charging ports?)

Lots of otherwise great sound equipment (ex. boomboxes, stereo receivers and digital pianos) has no built-in bluetooth transmitting capability and must therefore run sound their sound signals only through built-in or wired speakers.

Why not use otherwise scrap phones to transmit sound to portable bluetooth headphones and speakers?