Please note: The following only works on handsets before the Android 4.0 (Ice Cream Sandwich, or ICS) update. After that the audio menu option is disabled and cannot be accessed.
Are you finding the Samsung Galaxy S2 handset volume, even at maximum, too low? Or the same when connecting via the headset? Well, there’s a secret ServiceMod menu where you can set it.
On the phone keypad, type…
This enters the hidden ServiceMode screen. Be careful to not change anything in here unless you know what you’re doing. However, if you need to do a complete factory reset of the SG2, there is an option for that within here, so it’s worth knowing.
Anyway, once in ServiceMode select…
-  AUDIO
-  NB (VOICE CALL)
- Choose Handset, headset, Speaker (which ever you want higher volume)
-  VOLUME
-  SRC Speech RX Volume
You will now be presented with the current volumes for each of the 6 volume settings. To edit any of these select it and then press the menu key (to the left of the home button) and select “Key Input”. Type in the new settings and press Enter – it will save automatically. Don’t press your back key at any point or it will exit out of ServiceMode. Instead click the menu button and you will find a “back” option within that.
To give you an example, I’ve modified my headset volumes to the following…0_lvl : 72 1_lvl : 76 2_lvl : 80 3_lvl : 84 4_lvl : 87 5_lvl : 92