res/drawable/seek_bar_thumb.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright (C) 2011 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
-->
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_enabled="true">
        <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
            <!-- First item is the outer transparent sphere. -->
            <item>
                <shape
                    android:shape="oval"
                >
                    <size
                        android:width="16dip"
                        android:height="16dip"
                    />
                    <solid
                        android:color="@color/voicemail_playback_seek_bar_yet_to_play"
                    />
                </shape>
            </item>
            <!-- Second item is the inner almost-opaque sphere.
                 Seems to derive its size from the outer, a size element doesn't change anything.
                 Looks like using left, right, top and bottom on the item is best to fix inner sphere. -->
            <item
                android:left="3dip"
                android:right="3dip"
                android:top="3dip"
                android:bottom="3dip"
            >
                <shape
                    android:shape="oval"
                >
                    <solid
                        android:color="@color/voicemail_playback_seek_bar_already_played"
                    />
                </shape>
            </item>
        </layer-list>
    </item>

    <!-- Do not show the thumb when disabled -->
    <item android:drawable="@android:color/transparent" />
</selector>