Its easy. Just a heads up however, Sounds of the Show does not work in the Vita version.
First, you want to download the Broadcast Sounds of the Show Pack in the link above for the stadium specific sounds. It has every team's intro music (FSN, ESPN, RSN, SNY, etc) and every team's HR music (so Eznark, you'll get "Give me some Lovin" with the Bob Uecker call). Over time, people will update with other specific calls, such as stadium sounds for walks, etc. It also has all of the old soundtracks from MVP 04 and '05.
Adding Audio Files to Your PS3 Hard Drive
Connect an external source (i.e. USB flash drive or external hard drive) that contains audio files to your PS3 via one of the USB ports
From your PS3’s XMB menu navigate to the USB Device under the Music tab
Press Triangle and "Display All'
Navigate to the file you want to copy, press Triangle and select Copy
Repeat #4 for each file you want to use in Sounds of The Show
Adding Files to Sounds of The Show
Load MLB 12 The Show and navigate to Options > Sounds of The Show> Playlist Creator
Use the D-pad left/right to move between the left column and the right column.
The left column is your XMB Audio Library and displays all the audio files you have added to your PS3 hard drive. L2/R2 sorts the files by criteria, X tags a file into the playlist displayed in the right column. Files can be tagged into multiple playlists.
The right column is where files are added so that they can be played in menus or attached to in game events. While in this column L2/R2 changes the playlist type, files must be added to In-Game Cues to in order for them to be used to customize players or teams. X removes the file from the current playlist.
Jukebox Additions - Files that will play while navigating menus or during games if Music Mode is enabled.
In-game Music Cues - Where you will add most files. These can be attached to team specific stadium events and/or home team player specific events such as walk-up music or home run song.
Imported Chants - Files that when attached to events and played in game sound like the crowd is chanting them. Traditionally music doesn’t work so well here but a single voice or sounds recorded with mics under the User Recorder do.
Imported Yells - Same as chants but are played with just a single voice not the whole crowd.
Marking Files in Music Editor
Each file that was added to the In-Game Music Cues category can be marked at up to three different spots so the files can be played from different spots other then the very beginning.
In the left column select the file to be marked. X plays and stops the file.
Use the D-pad to move to the right column and move up/down to select one of the three marks you wish to edit.
While the file is stopped, L2/R2 rewinds or fast-forwards the mark.
X plays the file from the currently selected mark.
While the file is playing, [] can be used to drop the mark selected.
Attaching files to Players and Stadium Events
Edit Players allows you to add the marked files from the previous steps to player specific events. Select the player you want to edit, navigate to the Personalize tab, select the event, select the file, and select the mark you wish to play for that event.
Edit Teams allows you to add audio to specific in game events. Select the organization, select the MLB, AAA, or AA team. The top part of the window describes the event currently selected. Select the event you want to edit, select the file, and select the mark you wish to play for the event.
A heads up, as it took me a minute to figure it out, but once you go into the music editor, when you tab over to the right panel (where the music you add in goes ), you have to hit R2 to cycle through the destination you want the music to go. So for instance, if you want to add the Fox Sports Midwest Theme, you hit R2 until you get to "Stadium Specific Sounds" and then you just add in the music you want.
EDIT2:
Another heads up, when the player walkup music is released, you'll see something like this:
FurcalBeltranHolliday
So it will have those 3 players walk up tracks in one full track. So you'll have to go into the music editor and then mark your points for when you'll want that players music to start. So in the music editor you'll see where each song begins and ends, so you'll just mark those spots. Then when you go into the player editor, you'll see "Walk up Music" and you'll then select that and go to that players track listing. So if you go edit Beltran, you go to FurcalBeltranHolliday, and when you select the track, it will ask what mark you'll want to use to start the music. So you'll select the second mark, since Beltran is second on the list.
If it sounds confusing, let me know and maybe I can get a quick youtube vid. on how to do it. But once you start it, its pretty easy.
Sorry for the long post!