diff --git a/resources/uk/co/majenko/audiobookrecorder/icons/important.png b/resources/uk/co/majenko/audiobookrecorder/icons/important.png
new file mode 100644
index 0000000..2adca02
Binary files /dev/null and b/resources/uk/co/majenko/audiobookrecorder/icons/important.png differ
diff --git a/src/uk/co/majenko/audiobookrecorder/AudiobookRecorder.java b/src/uk/co/majenko/audiobookrecorder/AudiobookRecorder.java
index 01f2fee..18c02e3 100644
--- a/src/uk/co/majenko/audiobookrecorder/AudiobookRecorder.java
+++ b/src/uk/co/majenko/audiobookrecorder/AudiobookRecorder.java
@@ -796,6 +796,8 @@ public class AudiobookRecorder extends JFrame {
                 JMenuObject moveUp = new JMenuObject("Move Up", c);
                 JMenuObject moveDown = new JMenuObject("Move Down", c);
                 JMenu mergeWith = new JMenu("Merge chapter with");
+                JMenuObject lockAll = new JMenuObject("Lock all sentences", c);
+                JMenuObject unlockAll = new JMenuObject("Unlock all sentences", c);
 
                 for (Enumeration<Chapter> bc = book.children(); bc.hasMoreElements();) {
                     Chapter chp = bc.nextElement();
@@ -878,6 +880,30 @@ public class AudiobookRecorder extends JFrame {
                     }
                 });
 
+                lockAll.addActionListener(new ActionListener() {
+                    public void actionPerformed(ActionEvent e) {
+                        JMenuObject o = (JMenuObject)e.getSource();
+                        Chapter c = (Chapter)o.getObject();
+                        for (Enumeration<Sentence> s = c.children(); s.hasMoreElements();) {
+                            Sentence snt = s.nextElement();
+                            snt.setLocked(true);
+                            bookTreeModel.reload(snt);
+                        }
+                    }
+                });
+                unlockAll.addActionListener(new ActionListener() {
+                    public void actionPerformed(ActionEvent e) {
+                        JMenuObject o = (JMenuObject)e.getSource();
+                        Chapter c = (Chapter)o.getObject();
+                        for (Enumeration<Sentence> s = c.children(); s.hasMoreElements();) {
+                            Sentence snt = s.nextElement();
+                            snt.setLocked(false);
+                            bookTreeModel.reload(snt);
+                        }
+                    }
+                });
+                        
+
                 menu.add(moveUp);
                 menu.add(moveDown);
 
@@ -887,6 +913,9 @@ public class AudiobookRecorder extends JFrame {
                 menu.addSeparator();
 
                 menu.add(peak);
+                menu.addSeparator();
+                menu.add(lockAll);
+                menu.add(unlockAll);
                 menu.show(bookTree, e.getX(), e.getY());
             }
 
diff --git a/src/uk/co/majenko/audiobookrecorder/BookTreeRenderer.java b/src/uk/co/majenko/audiobookrecorder/BookTreeRenderer.java
index 86481f0..f5232de 100644
--- a/src/uk/co/majenko/audiobookrecorder/BookTreeRenderer.java
+++ b/src/uk/co/majenko/audiobookrecorder/BookTreeRenderer.java
@@ -14,6 +14,8 @@ public class BookTreeRenderer extends DefaultTreeCellRenderer {
             if (s.isLocked()) {
                 ret.setForeground(new Color(0x20, 0x00, 0x00));
                 ret.setIcon(Icons.locked);
+            } else if (s.getStartOffset() == 0) {
+                ret.setIcon(Icons.important);
             } else {
                 ret.setIcon(Icons.sentence);
             }
diff --git a/src/uk/co/majenko/audiobookrecorder/Icons.java b/src/uk/co/majenko/audiobookrecorder/Icons.java
index b415f56..d82a267 100644
--- a/src/uk/co/majenko/audiobookrecorder/Icons.java
+++ b/src/uk/co/majenko/audiobookrecorder/Icons.java
@@ -21,5 +21,6 @@ public class Icons {
     static public final ImageIcon locked      = new ImageIcon(Icons.class.getResource("icons/locked.png"));
     static public final ImageIcon appIcon     = new ImageIcon(Icons.class.getResource("icons/appIcon.png"));
     static public final ImageIcon star        = new ImageIcon(Icons.class.getResource("icons/star.png"));
+    static public final ImageIcon important   = new ImageIcon(Icons.class.getResource("icons/important.png"));
 
 }
diff --git a/src/uk/co/majenko/audiobookrecorder/Sentence.java b/src/uk/co/majenko/audiobookrecorder/Sentence.java
index 822b78f..e1ee121 100644
--- a/src/uk/co/majenko/audiobookrecorder/Sentence.java
+++ b/src/uk/co/majenko/audiobookrecorder/Sentence.java
@@ -121,7 +121,7 @@ public class Sentence extends DefaultMutableTreeNode implements Cacheable {
         storedAudioData = null;
 
         if (!id.equals("room-noise")) {
-            autoTrimSampleFFT();
+            autoTrimSamplePeak();
             if (Options.getBoolean("process.sphinx")) {
                 recognise();
             }
@@ -574,6 +574,7 @@ public class Sentence extends DefaultMutableTreeNode implements Cacheable {
 
     public void clearCache() {
         storedAudioData = null;
+        System.gc();
     }
 
     public boolean lockedInCache() {
@@ -582,6 +583,7 @@ public class Sentence extends DefaultMutableTreeNode implements Cacheable {
 
     public int findNearestZeroCrossing(int pos, int range) {
         int[] data = getAudioData();
+        if (data == null) return 0;
 
         if (pos < 0) pos = 0;
         if (pos >= data.length) pos = data.length-1;