One possible problem is distribution. It seems likely to me that smartphones and babies are more widely distributed than baby monitors. Due to segmentation of the baby products markets baby monitors appear to be upmarket.

For example, a quick check of Walmart.com for store available baby monitors near my zipcode has the cheapest monitor at ~$35.00 and the rest at $80-100+. Because the $35.00 monitor is not available in stock at all the Walmarts near my zipcode, distribution and segmentation mean new parents not enjoying the benefits of relatively lavish baby showers and other gifts face a substantial consumer purchase...more than 10 hours at minimum wage.

As described, a minimum viable Hablo probably might require <$5.00 of hardware...microphone, power supply, and wireless. So I'd say there might be an underserved market in the US. Outside the US, I'd expect the same.

This is of course a possible problem that might be solved. With the caveat that the shape of the baby monitor market might reflect absolute requirements for profitability...but I think that's more first world economic theory than reality on the ground.