In Shanghai, you can consider the following places to find services for customizing external sound cards:

1. Shanghai Charm Electronic Technology Co., Ltd.: This company focuses on the research and development and manufacture of computer karaoke and network karaoke audio equipment, including mixer, audio reverberator, USB external sound card, etc.

2. Shanghai Chuangshengxiang: Although the specific service items are not explicitly mentioned, it is used as.

3. Shenzhen Hualian Technology Co., Ltd.: Although this company is in Shenzhen, they provide customization services, including the customization of sound cards.

The above information is for reference only, and the specific situation may change. It is recommended that you consult in detail before going to these places.