Earphone development can be roughly separated in to three stages:
1) target profile definition
2) technical product development
3) shell design
Many factors need to be taken into account such as ...
- target sound signature and acoustic performance
- shape (size, ergonomics)
- price / target market
- cost of goods
- platform considerations
I usually create a target product profile incldss target performance criteria to meet and start development according to those. Sometimes I also to life cycle manage existing products, e.g. to cater changes in parts that are no longer available or increase flexibility of the platforms (eg. additional connector options).
Once I have defined the target I start development and for most new earphones this starts with experiments around the acoustic stage, so combinations of drivers, the crossover electronics and sound duct shape.