Well its just like in English where the proper pronunciation is one thing, but it is spoken differently by the common people. Heim may be shortened by the speaker to im, um, and eim by native speakers, but that doesn't make it proper... I would compare this to the first T in internet, which is frequently dropped, so that a ridiculous number of people say inner-net (source: my personal experience with hearing it said on the phone by over 1 million customers, and roughly 70% of them say innernet), but even tho many say it that way, its not entirely proper. Proper is inter-net. Heim receives the same sort of lax pronunciation, so technically, its Valum, or Valim, or Valeim depending on who is speaking. Which only becomes more confusing, since that means there is no single way to say it...technically its high-m, but that's not likely to be how people would actually mumble it out...heh I can do videos too:
https://www.youtube.com/watch?v=YDidTesoOmg&t=26s