All cables, no matter how short, lose some "power". But some cables can be miles long and not have a problem. Depends on what type of power they carry.
Yours is carrying low level DC, so there will be a drop of voltage. That is no problem since you are running the sound signal into an amplifier circuit of that stereo receiver. No problem with "power".
But cables for sound have impedance. This is a characteristic of electricity in transmission that smooths out the changes in amplitude (voltage change). This can make sharp high notes sound muddy or dull. But 10 feet should not be a problem.
What is a concern is your phone as a sound source. The digital part of its sound system is very highly engineered for good sound reproduction. Where it falls down is the analog portion in the earphones port of the phone. You are taking that out to an amplifier. If any lousy sound comes from the phone, you will make everything sound louder, including the faults of the phone's sound circuit.
Get a better sound source than that phone and earphone port and cable. One good one would be a computer that is on line and has a good audio chip that can push a signal out to a digital coax or optical (TOS link) cable to your stereo's digital sound port.
Working with high quality sound equipment in entertainment theaters since 1973. Electronics manufacturing engineer. Building computers for decades.