Hi, wanted to ship this to india, but couldn't find it in the list of countries you ship to. Kindly advise, Thanks.

For a single speaker application, a better option would be a good quality bluetooth speaker with a built in battery. Unless you lose mains power for more than ten hours, no worries.

Actually I would like to build it up by adding more Sonos Play 5 later.

Why is it that an online UPS is being recommended? Also why pure sine wave output would be recommended?

Will your suggestion of using the 350 VA UPS still hold good if I pair up one more play 5?

Yes, it would be ample; pure sine wave is recommended to be the best suited for all kinds of power supplies in audio kit; I have no idea if the 5 units have power supplies that can tolerate stepped sine wave output, and I have not tried it with that kind.

I believe that it takes an Online UPS to deliver pure sine wave, but I could be wrong.

I did a bit of search on Amazon India and found out this:
https://www.amazon.in/APC-Online-1000VA-Battery-SRC1KI/dp/B01LY9CAMQ/ref=sr_1_1?ie=UTF8&qid=1514110514&sr=8-1&keywords=APC+SRC1KI-IN+UPS

I couldn't find an online UPS (producing pure sine wave) at a lower cost.
Are all the online units priced likewise or am I missing something?

There will be cheaper ones than APC, but not by a whole lot if you want some quality. The problem is oversizing - 1000 VA for 2 Sonos speakers is a massive massive overkill. Even 250 VA should be ample for 2.

I have a 3 KVA online at home, but it supports everything in use except A/Cs, water heaters and refrigerators. Expensive, but some of the cost is repaid in extending equipment life. And convenience of always on pays for the rest.

It begs the question: Why Sonos? With a USD 1000 budget - and closer to USD 1300 if you will pay duties - for 2 speakers, there are many options that are battery + mains powered, that deliver very good results.

Can you recommend something great?

I have a 3 KVA online at home, but it supports everything in use except A/Cs, water heaters and refrigerators. Expensive, but some of the cost is repaid in extending equipment life. And convenience of always on pays for the rest.

Did you mean an inverter with external battery?

In Amazon India I have seen that most of the models listed as UPS have an external battery. They usually have much more power back up (2-3 hours usually for lights, fans, other electrical equipments).

In Amazon US, all the models have inbuilt batteries but unfortunately they do not ship to India.

An example of such an UPS (or inverter?) in Amazon India:

https://www.amazon.in/gp/aw/d/B009Y4BSAE?psc=1

Coupled with a decent battery the cost would come to about 18000INR.(280 US dollars).

Are you talking about similar products? I am very confused, indeed.

Where the battery is located is relevant only for the back up time available. I have a DG set back up, so my UPS needs just a battery capacity of 10 minutes to support the switchover interval of 2-3 minutes and to deliver exactly 230V50Hz, 24/7. So a built in battery is enough for this application. Extended back up time needs external batteries.

I also see this for a much lower price!: https://www.amazon.in/dp/B009Y4BSAE/ref=sspa_dk_detail_0?psc=1

I do not suggest buying this online, but from a reputed dealer in your city because you need to make sure you are getting just what is needed for the right price. So, some consultation may be needed.