1) FMS add volume keys (taken from Clip keymaps)
static const struct button_mapping button_context_radio[]

    { ACTION_SETTINGS_INC,             BUTTON_VOL_UP,                    BUTTON_NONE },
    { ACTION_SETTINGS_INCREPEAT,       BUTTON_VOL_UP|BUTTON_REPEAT,      BUTTON_NONE },
    { ACTION_SETTINGS_DEC,             BUTTON_VOL_DOWN,                  BUTTON_NONE },
    { ACTION_SETTINGS_DECREPEAT,       BUTTON_VOL_DOWN|BUTTON_REPEAT,    BUTTON_NONE },

2) Fix/improve the keyboard (playlist or preset creation etc)
static const struct button_mapping button_context_keyboard[]

Replace
    { ACTION_KBD_SELECT,                BUTTON_POWER,                      BUTTON_NONE },
    { ACTION_KBD_DONE,                  BUTTON_PLAY|BUTTON_REPEAT,         BUTTON_PLAY },

by
    { ACTION_KBD_SELECT,                BUTTON_PLAY|BUTTON_REL,            BUTTON_NONE },
    { ACTION_KBD_DONE,                  BUTTON_PLAY|BUTTON_REPEAT,         BUTTON_NONE },
    { ACTION_KBD_ABORT,                 BUTTON_POWER,                      BUTTON_NONE },