#!/bin/bash
s="There is stuff here (word, number, phrases) , even more :) with smilies. The quick, (brown, fox) jumps over, the (lazy, old, dog)"
echo $s | sed '/(/ {:a s/\(([^,()]*\),/\1:/; t a}'
echo $s | sed '{:a;s/\(([^,()]*\),/\1:/;ta}'
echo $s | sed -E '{:a;s/(\([^,()]*),/\1:/;ta}'