Intermittent drops in audio level followed by distortion

I am running Windows 7 on an HP DM4 Pavilion Beats Edition. I had no problem playing mp3s or mp4s until recently. I started to find that the volume level would suddenly reduce quite drastically and after a few seconds the audio would become severely distorted while still at reduced level. This would sometime last for several minutes or maybe just a few seconds. The volume level would suddenly be restored and the audio would be clear as a bell again. I have tried checking out all the audio settings I know about as well as the Beats IDT ones without any improvement. I have not only reinstalled the drivers but downloaded fresh versions although all the drivers involved are up to date versions. I have listened on another laptop to copies of mp3/mp4s on which I heard the distortion and they were perfect. I have downloaded a different mp3/mp4 player and the problem was still there. I know there were problems with upgrades to windows 10 and distortion but there is little relating to Windows 7. The strange things is that the sound was perfect until fairly recently.

Process Lassoo does not show any spikes when the distortion occurs and I can't see any other program suddenly cutting in at that time either. I know some people have had distortion problems with Realtek sound cards but my laptop doesn't have a plug in sound card just the default Beats set up. Other than the usual windows 7 updates I haven't had anything which could affect the audio (as far as I am aware). There have also not been any hardware changes. Video playback does not seem to have been affected in any way. All this just came out of the blue and now I can't enjoy listening to music!

It seems to me as if there is some kind of conflict within windows 7 but I don't know what. I really don't want to have to reinstall Windows 7 given no installation disk came with the laptop since all that information resides on a partition on the HD. I would be grateful if anyone who has experienced a similar problem could give me the benefit of their experience and how they fixed it.

Further to my earlier post I should have added that running SFC / ScanNow made no difference. At the moment the sound is quite low and the distortion very bad on all of the news channel videos and audio. I ran the sound test on each of the Beats internal speakers and the sound is as clear as a bell. So it is not speaker hardware related.

I uninstalled Skype but that made no difference. I have also just checked that there are no IRQ conflicts. Everything is reported as being in order, except it's not!