First add this name space : 
using System.Runtime.InteropServices;
Now you can simply disable all audio output. Try these codes :
[DllImport("winmm.dll")]
    public static extern int GetVolume(IntPtr p, out uint volume);
    [DllImport("winmm.dll")]
    public static extern int SetVolume(IntPtr p, uint volume);
    public Form1()
    {
        InitializeComponent();
    }
    private void Form1_Load(object sender, EventArgs e)
    {
        // Save the current volume
        int save;
        GetVolume(IntPtr.Zero, out save);
        this.FormClosing += delegate 
        {
            // Restore the volume 
            SetVolume(IntPtr.Zero, save);
        };
        // Now you can mute sounds
        SetVolume(IntPtr.Zero, 0);
        string url = "http://www.example.com";
        this.webBrowser1.Navigate(url);
    }
Update :
You can Read This someone else also answered this question. 
Update 2 :
You can put it inside a static class and either make the CoInternetSetFeatureEnabled method public, or add an additional bridge method that calls it after converting the parameters from a more usable form, if necessary.
Read these two similar questions and disable the sound :
Question1 Question2
Update 3 :
For IE7 and above you can use CoInternetSetFeatureEnabled :
// Constants
private const int FEATURE_DISABLE_NAVIGATION_SOUNDS = 21;
private const int SET_FEATURE_ON_THREAD = 0x00000001;
private const int SET_FEATURE_ON_PROCESS = 0x00000002;
private const int SET_FEATURE_IN_REGISTRY = 0x00000004;
private const int SET_FEATURE_ON_THREAD_LOCALMACHINE = 0x00000008;
private const int SET_FEATURE_ON_THREAD_INTRANET = 0x00000010;
private const int SET_FEATURE_ON_THREAD_TRUSTED = 0x00000020;
private const int SET_FEATURE_ON_THREAD_INTERNET = 0x00000040;
private const int SET_FEATURE_ON_THREAD_RESTRICTED = 0x00000080;
// Necessary dll import
[DllImport("urlmon.dll")]
[PreserveSig]
[return:MarshalAs(UnmanagedType.Error)]
static extern int CoInternetSetFeatureEnabled(
int FeatureEntry,
[MarshalAs(UnmanagedType.U4)] int dwFlags,
bool fEnable);
......
// You can call the CoInternetSetFeatureEnabled like this:
CoInternetSetFeatureEnabled(FEATURE_DISABLE_NAVIGATION_SOUNDS, SET_FEATURE_ON_PROCESS, true);
Here is the Source
Update 4 :
How to mute windows sound