[AT&T, Rogers, Bell, Telus Samsung Galaxy S III] loserskater:
Already implemented and just want to add the setting?

Follow the SecSettings and PhoneWindowManager$SettingsObserver.smali additions in the OP.

Now here are the changes/additions that need to be made in PhoneWindowManager.smali:

RED is new code

GREEN is code from the first change

.field mVoiceTalkDefaultLaunch:Z

.field mVoiceTalkIntent:Landroid/content/Intent;

[COLOR="red"].field mVolBtnMusicControls:I[/COLOR]

.field private mVolumeDownKeyConsumedByScreenshotChord:Z

method interceptKeyBeforeQueueing:

const-string v23, "WindowManager"

const-string v24, "ITelephony threw RemoteException"

move-object/from16 v0, v23

move-object/from16 v1, v24

invoke-static {v0, v1, v6}, Landroid/util/Log;->w(Ljava/lang/String;Ljava/lang/String;Ljava/lang/Throwable;)I

.line 5333
.end local v6 #ex:Landroid/os/RemoteException;
:cond_20
move-object/from16 v0, p0

[COLOR="green"]iget-boolean v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mScreenOnEarly:Z

if-nez v0, :cond_reg

invoke-virtual/range {p0 .. p0}, Lcom/android/internal/policy/impl/PhoneWindowManager;->isMusicActive()Z

move-result v21

if-eqz v21, :cond_reg

[COLOR="Red"]const/16 v1, 0x1

move-object/from16 v0, p0

iget v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mVolBtnMusicControls:I

if-ne v0, v1, :cond_reg[/COLOR]

move-object/from16 v0, p0

move-object/from16 v1, p1

invoke-virtual {v0, v1}, Lcom/android/internal/policy/impl/PhoneWindowManager;->handleVolumeLongPress(Landroid/view/KeyEvent;)V

:cond_reg
move-object/from16 v0, p0[/COLOR]

iget-object v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mSamsungVolumeControlThread:Lcom/android/internal/policy/impl/PhoneWindowManager$SamsungVolumeControlThread;

move-object/from16 v23, v0

if-nez v23, :cond_1

and a couple lines down:

move-object/from16 v23, v0

invoke-virtual/range {v23 .. v23}, Lcom/android/internal/policy/impl/PhoneWindowManager$SamsungVolumeControlThread;->start()V

goto/16 :goto_2

.line 5341
.end local v22 #telephonyService:Lcom/android/internal/telephony/ITelephony;
[COLOR="Blue"]:cond_long[/COLOR]
[COLOR="green"]if-nez v5, :cond_vc

move-object/from16 v0, p0

iget-boolean v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mIsVolumeBlocking:Z

if-eqz v0, :cond_vc

[COLOR="red"]const/16 v1, 0x1

move-object/from16 v0, p0

iget v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mVolBtnMusicControls:I

if-ne v0, v1, :cond_vc[/COLOR]

invoke-virtual/range {p0 .. p0}, Lcom/android/internal/policy/impl/PhoneWindowManager;->handleVolumeLongPressAbort()V

move-object/from16 v0, p0

iget-boolean v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mIsVolumeAction:Z

if-nez v0, :cond_vc

move-object/from16 v0, p0

const/4 v3, 0x0

invoke-virtual {v0, v3, v15}, Lcom/android/internal/policy/impl/PhoneWindowManager;->handleVolumeKey(II)V

:cond_vc[/COLOR]
move-object/from16 v0, p0

iget-object v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mSamsungVolumeControlThread:Lcom/android/internal/policy/impl/PhoneWindowManager$SamsungVolumeControlThread;

move-object/from16 v23, v0

if-eqz v23, :cond_1

method updateSettings:

const-string v19, "incall_power_button_behavior"

const/16 v20, 0x1

move-object/from16 v0, v19

move/from16 v1, v20

invoke-static {v13, v0, v1}, Landroid/provider/Settings$Secure;->getInt(Landroid/content/ContentResolver;Ljava/lang/String;I)I

move-result v9

[COLOR="red"]const-string v19, "volbtn_music_controls"

const/16 v20, 0x0

move-object/from16 v0, v19

move/from16 v1, v20

invoke-static {v13, v0, v1}, Landroid/provider/Settings$System;->getInt(Landroid/content/ContentResolver;Ljava/lang/String;I)I

move-result v3[/COLOR]

.line 2006
const-string v19, "user_rotation"

iput v6, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mEndcallBehavior:I

.line 2028
move-object/from16 v0, p0

iput v9, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mIncallPowerBehavior:I

[COLOR="red"]move-object/from16 v0, p0

iput v3, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mVolBtnMusicControls:I[/COLOR]

.line 2030
move-object/from16 v0, p0

iget v0, v0, Lcom/android/internal/policy/impl/PhoneWindowManager;->mUserRotation:I