android:gravity
handles the alignment of its children,
android:layout_gravity
handles the alignment of itself.
So use one of these.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#000"
android:baselineAligned="false"
android:gravity="center"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Main" >
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:gravity="center" >
<ImageView
android:id="@+id/imageButton_speak"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/image_bg"
android:src="@drawable/ic_speak" />
</LinearLayout>
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:gravity="center" >
<ImageView
android:id="@+id/imageButton_readtext"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:background="@drawable/image_bg"
android:src="@drawable/ic_readtext" />
</LinearLayout>
...
</LinearLayout>
or
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#000"
android:baselineAligned="false"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".Main" >
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_weight="1" >
<ImageView
android:id="@+id/imageButton_speak"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/image_bg"
android:src="@drawable/ic_speak" />
</LinearLayout>
<LinearLayout
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:layout_weight="1" >
<ImageView
android:id="@+id/imageButton_readtext"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:background="@drawable/image_bg"
android:src="@drawable/ic_readtext" />
</LinearLayout>
...
</LinearLayout>