Door location announcement

I've been wondering about this my entire holiday. In some of the trains, they might announce the location of the the opening doors as (what sounds like) this:

お出口は左だわです。
(romanized for novice readers: odeguchi wa hidari dawa desu)

I'm not sure about the だわ part. It sounds like that's what they say. But I have no idea what it could mean. Iyam, お出口は左です should mean exactly the same thing, right?

I can't find the meaning of だわ, or other words that sound alike, anywhere with an applicable meaning. Neither as a noun nor as a particle of some kind.

So:
1) What is it exactly what they add before です? Is it だわ or did I mishear?
2) What meaning does it add to the sentence?

I believe they are saying either:
降り口は左側です　（おりぐちは　ひだりがわ　です）
or
お出口は左側です （おでぐちは　ひだりがわ　です）

The part you are wondering about is definitely "gawa." "Hidari gawa" means "left side."

おでぐちは ひだりがわ です（お出口は左側です）
The doors on the left side(toward to the next station) will open

You have already the correct answer.
Supposing the reason of your miscomprehension, I guess you don’t distinguish, in this case, the difference between two g sounds (g-/-ga, -ga-.
In the spoken Japanese of the Tokyo region, which is regarded now as so-called standard or hyoujungo, or NHK like Japanese, -g and -g- are pronounced like the g sound of English -ing, but g- of the head of word is pronounced like the ordinary g sound. In the other regions of Japan, especially in the Kansai region, and the younger generations are, this pronounciation is not practiced.
You confused -nga with -da.

Yes, that is "gawa(側)" means just "side" as small emphasis(強調) and polite(丁寧).
Train car annoucement is special speaking style for noisy packed cars passengers,
so voices make muffled as custom. (woman conductors voices are not so strong)
(also automatic annoucement machine=recorded voices are not so strong)

側(kawa,gawa/soku) is basically side of right or left mainly as side faces "側面(soku-men)",
https://en.wiktionary.org/wiki/%E5%81%B4
but many more various explanations words usable.
https://goo.gl/8PAEUu
上側(ue-gawa)/下側(shita-gawa)
あちら(あっち)側/こちら(こっち)側/そちら(そっち)側/どちら(どっち)側
向(mu)こう側/向かい側
外側(soto-gawa)/内側(uchi-gawa)
表側(omote-gawa)/裏側(ura-gawa)
片側(kata-gawa)/両側(ryo-gawa)
山側(yama-gawa)/海側(umi-gawa)
ホテル側(hotel-gawa)/駅側(eki-gawa)

When indicating direction or location,
many cases will be replaced with "の方(no-hou)/方(hou)" or "寄り(-yori)".
左側/右側 = 左の方/右の方, 左寄り/右寄り even 左肩(-kata)/右肩,
上側/下側 = 上方(jo-hou)/下方(ka-hou), 上の方/下の方, 上寄り/下寄り
東側/西側 = 東方(tou-hou)/西方(sai-hou), 東(higashi)の方/西(nishi)の方, 東寄り/西寄り
南側/北側 = 南方(nan-pou)/北方(ho-ppou), 南(minami)の方/北(kita)の方, 南寄り/北寄り
片側/両側 = 片方/両方


And, this no related about "gawa",
some rail companies dosen't use "お出口/おでぐち", only "出口" like this.
-----------------------------------------------------------------------------
次は、国会議事堂前、国会議事堂前です。
乗り換えのご案内です。　丸ノ内線、南北線、銀座線は お乗り換えください。
The next station is Kokkaigijido-mae.
Please change for the Marunouchi Line, the Namboku Line and the Ginza Line.
足元にご注意ください。電車とホームの間が広く空いております。
出口は、右側です。
-----------------------------------------------------------------------------

How to use "-dawa/-だわ"?
This is Woman phrases(= わ end style)
and as emphasis or assertion(断定) = だ(including だった as past) mainly.　
Even Man uses it though instead of -だな or -だぜ like this.
https://goo.gl/RSB36v
(There are many of other endings in dialects.)
------------------------------------------------------------
@ ... (guest)
I don't think so, about process of miscomprehension.

If so, he(?) would have mistakenly heard it.
A lot of "ga-" words "gakko/学校", "gaikokujin/外国人" and more,
https://dictionary.goo.ne.jp/srch/jn/%E3%81%8C/m0p100u/
even "xx - ga, xxが、" as 格助詞 this is one of super basic letter.
https://en.wikipedia.org/wiki/Japanese_particles

to OP,
that is not your mistake.
https://www.youtube.com/watch?v=kbAPzYIraxA
I can't catch "ga" pronunciation. it can be heard as hidari-(ng)awa. very strange.

this is the normal pronunciation of "hidari-gawa" by Japanese.
https://www.youtube.com/watch?v=Jlf19L1S1hI

I forgot to write in my previous comment; this light ng- like sound is called 'a voiced velar nasal consonant'; in Japanese more simply, びだくおん（鼻濁音）.
We have no official letters in our Hiragana and Katakana systems to represent this sound. But, it exists.
See this link:
https://ja.wikipedia.org/wiki/鼻濁音
(No English version, but the OP seems not to be a novice, it will be no problem.)
And read my previous comment, pay attention to the hyphens in my comment: g-/-g, -g-.