Will a transformer, 220 to 100v, ruin the sound?

I think it depends on the quality of the transformer, but the advise I got going the other direction was yes, it could. I live in the UK and recently bought a Conrad Johnson Premier 17 pre (from Audiogon of course). I had to convert 230 to 110.
A word of caution, you do'nt always have to change the transformer. Many companies make products easily adjustable, they are trying to meet an international market after all. I contacted the CJ importer in the UK, after much sucking of teeth and tutting, he said he could adjust the voltage for £300 ($500!). I took it to an electronic engineer who simply adjusted the connections to the transformer, as the CJ was designed to do, 1 hour and £25.
Just make sure you can'nt adjust the voltage internally before going for external transformers. Another friend spent $3000, for a fancy JPS transformer for his imported Spectral 12 pre and when sent for repair, there was a simple switch inside to adjust the voltage

This question keeps coming up, so a word of explanation may be in order.

A transformer consists of many turns of wire, the "Primary", wound around an iron core and connected to the input power. Also wound on the iron core is another wire,the "Secondary". When voltage is applied to the Primary, a voltage will be generated from the Seconary. The Secondary voltage level depends on the "Turns Ratio", as well as the voltage applied to the Primary. If the Primary has 400 turns and the Secondary has 300 turns, the ratio is 3/4. If the voltage applied to the primary is 110 VAC, the Secondary voltage will be 0.75*110, which is 82.5 VAC. Let's say that this is what the amp circuit needs.

If you applied 220 VAC, instead of 110, the Secondary would generate 165 VAC, which would zap the circuitry!

However, the transformer Primary can the wound so that there are two Primary windings, each having 400 turns.

If the 110 VAC input is appled to only one of these Primary windings you will get the needed 82.5 VAC from the Secondary. In practice the two primary windings are both used, connected in parallel, which has the same effect as using larger wire for the windings, so that more power (watts) can be generated.

If the two primary windings are connected in series, the total number of Primary turns is 800, and the turns ratio is 0.375. When 220 VAC is applied to the series-connected primary the seconary voltage will be 0.375*220 which is the necessary 82.5 VAC.

All of this is just to make clear how trivial a change it is to modify equipment for different input voltage. It has been many years since I saw equipment that lacked dual Primary windings in the transformer. The global economy demands it. All that the equipment manufacturer needs to do is include a switch to change the transformer Primary windings from parallel to series connection. Sometimes this switch is under the cover, and sometimes it is external. Sometimes the "switch" is actually a little jumper connector that you move from one socket to another. In the worst case the transformer wires might be hard soldered in one configuration or the other...still a minimal task with a soldering iron.

DON'T LET ANYONE SELL YOU AN EXPENSIVE EXTERNAL STEPUP OR STEPDOWN TRANSFORMER. AND DON'T LET ANY UNSCRUPULOUS TECHNICIAN CHARGE YOU AN ARM AND A LEG TO THROW THE SWITCH!

Listen to Eldartford...he is wise!

Changing the transformer taps on HiFi equipment often works fine, just like Eldartford says. There are some exceptions though. Some high end equipment manufacturers (Krell for example) now install a mains frequency sensing chip to prevent US gear (110V, 60Hz) being freely imported into Europe (~230V, 50Hz) and simply used with a step down transformer (220-240V down to 110V). The equipment simply will not work if you do this (I found out the hard way when I took my Krell HTS to Europe for a temporary overseas assignment). These manufacturers want you to buy the equipment in Europe in the first place, or pay them (many $$$) to do the conversion for you. See my previous post on this subject. Be warned.

I was told by Tom Tutay, a well respected tube amp tech, that a step up or step down transformer would better isolate the piece of equipment in question from the ac line and would thus be a good thing. I never tried it so I don't really know.