Electricity fluctuations

Do you have a transformer on the power pole at the end of your 100' line? Or are you using the same transformer as your neighbor? If the latter is the case, you can request your power co to put your own transformer up for you - at their cost. I did this and cured the same problem you are having. In power co. lingo it's called "a too big line drop" and it is technically not allowed. You may have to do some talking and calling to get it though.... The fact that you are ok at night supports this (although power is generally smoother at night anyway).

Do you have a heat pump? If so, check to make sure it is well grounded. Mine would cause line dips when it came on. See if you can time the fluctuations with something like that coming on (water well pump can do it too) so you can isolate the problem. Good luck!

assuming your main panel and associated incoming cabling are appropriately sized for the loads you have then you should not experience more than a 10% voltage swing under "normal" circumstances for your local utility.
if you have a multimeter you should record incoming line voltage at various times. if you are experiencing voltage swings like you described (~40%) there is no utility in the country that isn't legally bound to correct the problem (assuming you don't have the problem).
if these conditions existed since installation then a severely overloaded transformer may be the problem. if it is a problem that has developed over time then i would suspect something is failing. it is not uncommon for overhead lines that are run through trees to eventually have a limb contact the line and through wind action eventually rub away the insulation in one spot and cause voltage fluctuations. the utility can verify the integrity of the incoming lines with a simple megger test.
call the utility and see if you get an appropriate response. if not then definately put it all in writing and send a certified letter.

The power company is partially to blame but fat chance they'll help out. If you are at the end of a rural line, there is a good chance that your problem of voltage sag is caused by power line faults from things like tree branches, damaged power poles, animals, birds, rain, snow, etc. These faults reduce the grid voltage downstream of the fault - and since you are the most downstream user, you see the fluctuations. These can be corrected by power company maintenance but rarely do they have the manpower to cover the large real estate on a daily basis.

One way of solving the voltage sag is to install an electronic sag protector or dip proofing inverter. These are, in essence, power supply capacitors with an ac inverter. Too expensive to do it for the whole house. Instead, install a subpanel from the main service panel and install the inverter in the subpanel. The size of the inverter should be 2 kVA per circuit. Keep it simple - a subpanel with 3 dedicated circuits and a 6 kVA single-phase inverter should solve your problem. The other way of solving this is to install a generator (too costly and noisy).

Another problem you may face is the 100 ft of buried service conductor. It may be too small. If your service is 100 amps, the size of the conductor should be 1/0 (three sizes larger to accomodate the voltage drop and the derating from burial). Have an electrician confirm, and, while he's at it, check the grounding to see if it's less than 25 ohms. He should also check to see that all phase conductors in the panel are tight. Best of luck...

Even if the line is "hard" at your transformer, your 100' drop coming to the house may be too small a wire gauge or may have bad connections, resulting in excess voltage loss during high current demand (heat pump running). Use a known-calibrated voltmeter to measure both 120V legs incoming to your distribution (fuse or breaker) box in order to find out. Comparable measurent at the load end of your dedicated audio circuit won't read much, if any lower, due to that large #8 gauge wire. But if you can access any measurement points right after the transformer, before the power enters that 100' drop cable,(being very careful, or better, having the power company measure this if you're unsure regarding safety issues) then comparing the at-house voltage against the at-transformer voltage will tell you if that 100' drop cable is too small. Or it will also tell you if the voltage out of the transformer is too low. In either case the power company has to take corrrective actions. But typically they won't do anything for you unless you supply measured evidence of a problem. And even at that, you'll probably have to make your valid complaints known to an account management supervisor before they'll do anything about it, based upon my experience with the glacial speed of power providers. Their way of thinking "if the lights are on then everything's fine" has been my unfortunate experience.

Due to work I did not get to read answers to my "electricity fluctuations" untill saturday eve. I thank you very much for the time you took to send me those detailed instructions, Aball, Lwatson, GS5556, and bob bundus. Now I need sometime to investigate the problem according to your suggestions.
Best regards
Theo