Hoenn and Sinnoh Sounds
The Hoenn and Sinnoh Sounds are remixes of music you are familiar with when playing through the first routes in each of those regions. The remixes can only be played once a week and you will first need to have obtained the National Dex to hear them. Where do they play, you may be wondering...
You will need to tune into the Pokémon Music Channel on Wednesday and Thursday, using your PokéGear's radio function, to hear Hoenn and Sinnoh's remixes respectively. With that in mind this particular mechanic, while it may be useful to some or even many, has its drawbacks.
Being that radio stations are part of the PokéGear, this method of catching Pokémon is heavily flawed. Why flawed? If the purpose of tuning into a particular station was to hear whatever was playing, you might expect it to play without much interruption. Emphasis on much, provided things like trainer battles or plot events take precedence over whatever selected soundtrack is playing. However, the station currently playing is lost doing even the most basic of tasks and as such using the Pokémon Music Channel to encounter these rare Pokémon is a lot more of a chore than could be worth it, especially if you have other means of obtaining these Pokémon.
At any rate, onto the information you are probably looking for...
The Hoenn Sound is a remixed version of the Rt. 101 theme. It plays on Wednesdays when tuning into the Pokémon Music Channel on your PokéGear. The Pokémon found with this music playing are all, obviously, native to Hoenn. I have paired them based on the similarity of the appearance locations and listed all of the locations that they can be found at.
The Sinnoh Sound can be heard playing on Thursdays on the Pokémon Music Channel. It plays a remixed version of the Rt. 201 theme and this attracts Pokémon that are native to Sinnoh (and are generally found early on within that region). I have paired them up because their appearances are similar across all of the locations they can be found when the Sinnoh Sound is playing.