VC++ 2005 Express - LNK2001 with static var

My C++ is pretty rusty so this might just be a stupid oversite but,

I'm trying to build a DLL with the following class:

//************** SoundObject.h ***************
#ifndef SOUNDOBJECT_H
#define SOUNDOBJECT_H

#define DllExport __declspec(dllexport)

typedef double SoundChunk; // This type is used by most objects in the
SoundTools library
namespace SoundTools
{
    class SoundObject
    {
    private:
        static SoundChunk _sampleRate; // default sample rate
    protected:
        SoundObject();
    public:
        static DllExport SoundChunk GetSampleRate(void);
        static DllExport void SetSampleRate(SoundChunk r);
    };
}
#endif

//************** SoundObject.cpp ***************

#include "SoundObject.h"
namespace SoundTools
{
    SoundObject::SoundObject()
    {
        SoundObject::_sampleRate = 44100.0;
    }
    SoundChunk SoundObject::GetSampleRate()
    {
        return _sampleRate;
    }
    void SoundObject::SetSampleRate(SoundChunk r)
    {
        if(r > 0.0)
            _sampleRate = r;
    }
}

//********************************************

When I try to build the DLL I get this linker error:

SoundObject.obj : error LNK2001: unresolved external symbol "private: static
double SoundTools::SoundObject::_sampleRate"
(?_sampleRate@SoundObject@SoundTools@@0NA)

I tried some different things including declaring _sampleRate as extern and
using explicit declarations in the *.cpp file. Not sure what I'm doing
wrong.