My two are 15 months, and although they do both walk well, there are almost no words here yet either, apart from 'da' - there and 'yes'. They do a very firm headshake for 'no' as well.
Looking at their peer group, I don't think they're especially slow, just some walk and talk later than others. They were the 3rd & 4th in our NCT group to walk, and speechwise I think only 2 of our group have any speech yet.
Try doing some baby signing with them You don't have to go to expensive classes, you can buy a book, look it up online, or even just make up your own. We did just the basic things like milk, food, drink, more, all gone, tired. That was enough to get them understood. In my experience it really encouraged ours to talk as they realised they could make themselves understood with the signs (stopped a lot of the crying!), then would do the sign and attempt to say the word, then the sign was dropped. We also did all the fun signs, like all the animals etc so we could do them along with stories/songs. It's great fun :-)
18 month olds here. One walked at 15 months, one at 16 - now they are charging about like all their friends who were walking by their first birthday. After nearly a year of signing at them they are suddenly using them and picking up new ones straight away. Actual words? None. Noises that mean specific things (to me and DH)? A few. The clearest is "nap nap" for crocodile. Very useful in everyday life . Think one of them said Mama actually meaning me (rather than "I want") yesterday. That was very nice.
my twin boys have just turned 16 months. They were earlyish to start walking at 11 months and 12 months but really dont say much at all yet. They say dada, dare (there) ba (ball) da (dog)and thats about it. I can sometimes get a mama out of them but i have to work at it. They can make animal noises when i show them books, duck, cow, dog, etc. They babble a bit through the day, squeal, etc but really they are pretty quiet. I alternate between sometimes worrying about it and other times relaxing and knowing that it will come in its own time.